In the summer of 1978, six months after John had left Japan, the three decided to take a trip on Kerry’s yacht, Foxy Lady. They set off on a course towards Bangkok, three experienced sailors on a relatively short hop across the water. John, a keen photographer, took pictures of islands along the way, splitting his time between navigating and cooking simple food below deck.

Then they vanished without a trace.

On the other side of the world from Hilary, in New Zealand, Kerry Hamill’s brother Rob had also received a final letter, in “either June or July of 1978”. Kerry’s girlfriend, Gail Colley, had mostly been on the boat throughout the year, but she had flown out to Hawaii to visit her parents in June, and returned to meet up with the boys a few weeks later, but they did not show up.

For the Dewhirsts and the Hamills, periods of no communication from Kerry and John were normal. It was still relatively rare for young people to set out on their own for Southeast Asia; the places they visited were not the well-worn backpacker trails of today. The internet was decades off, and mail delivery across continents was slow. But as time went on and no letters arrived, the families began to worry.

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The Hamills had, in preceding months, sat together around the kitchen table as their father read out the latest letter from Kerry vividly describing his adventures; sometimes an item of clothing or a trinket would arrive in a package for his siblings. Weeks without Kerry’s reassuring words went by. His mother would stare out to sea whenever anyone expressed anxiety and brightly say, “Don’t you worry. He’ll turn up and surprise us in time for Christmas.”

“It was 16 months before we knew what happened to him,” Kerry’s younger brother Rob Hamill tells me from his own boat in New Zealand (he is also a keen adventurer, won the first Atlantic Rowing Race, in 1997, and competed in the 1996 Olympics). “My mum and dad were beside themselves; my dad had been contacting every single port in Southeast Asia, trying to find out something, anything. They had such hope and fear.” He pauses. “I didn’t understand it completely then, but now I have children of my own.”

The first time I came across the name John Dewhirst was on a trip to Phnom Penh, in the rainy season of 2011, running my fingertips along a row of faces and documents at Tuol Sleng Genocide Museum. Originally a high school, from 1975 until 1979 the compound was an interrogation centre – known as Security Prison 21 (S-21) – of the ultra-Maoist Khmer Rouge regime under the leadership of Pol Pot.

I had stopped in front of an unexpected typed confession, one which mentioned the streets, schools and shops of my own childhood in Newcastle, in the north of England.

“My name is John Dawson DEWHIRST,” the confession read. “British national … I’m a CIA agent. Recently I was disguised as a teacher in Japan. I was born on 2 October 1952, in Newcastle, England … My father was also a CIA agent. He had a legal position as principal of Benton Rov Secondary School … My mother is a secretary in a business store named Fenwick … In 1957, I went to West Jameson Junior School in Newcastle. In 1962, I was transferred to a private school called Royal Grammar School in Newcastle. I moved because my father thought Royal Grammar was a better school than West Jameson.”

To anyone who grew up in Newcastle, these names (Fenwick’s, the misspelt Benton Road, the school known colloquially as RGS) are familiar. I could not understand how someone who had grown up in my neighbourhood had ended up with their supposed CIA confession and photograph pasted on the wall of a torture centre just outside the Killing Fields of Cambodia. It didn’t make sense.

Tuol Sleng Genocide Museum is a harrowing place, its numbered concrete cells, with their dusty, chequered floor tiles, standing just as they did during the height of the Khmer Rouge. Iron beds, rusted shackles attached to their frames, and implements used to kill and maim remain in rooms where torture was carried out. Dried blood marks walls. This is no sanitised memorial, and today, photographs of prisoners – left behind when S-21 interrogators fled from the invading Vietnamese army in the final days of the regime, in 1979 – bear witness to the extent of the atrocity.

Thirty minutes away by car is the best known of the sites known as the Killing Fields, the Choeung Ek Genocidal Centre. S-21 inmates were transported here after their confessions had been written up, rewritten and perfected by the head of the prison, the ruthless Khmer Rouge operative and former maths teacher Kang Kek Iew, better known as Comrade Duch (pronounced Doik). At Choeung Ek, visitors today are met with rows of human skulls dug from the mass graves into which slaughtered people were thrown for crimes that included being able to speak French, wearing spectacles or loving a family member “too much”.

Such was the scale of the killing at Choeung Ek that visitors are urged to keep to wooden pathways that curve around the large depressions that are mass graves. Areas where bodies are yet to be excavated are cordoned off with wooden fences. One is known to be the mass grave of more than 100 women and children; another is labelled “450 victims”. In some areas, visitors are warned by signs in Khmer and English that small fragments of bone and teeth are likely to appear after heavy rain.

A wide tree trunk, on which children were beaten to death, is pointed out; the music that guards used to cover up screams is played loud. Around the perimeter of the fields, limbless beggars – victims of landmines – politely inquire whether you might have any US dollars.

“I was told a story by a guard when I visited there,” says Hilary, “that John was taken out, tied into a tyre and set on fire at S-21. But he didn’t know if it might have been a different Westerner.”

It is a story that I encounter again and again in my research, until one rare survivor of the prison – a Cambodian man named Chum Mey – tells me that he was detained in the room next door to Kerry Hamill, and that Kerry was the one who met that awful fate. It is unclear what provoked this act of brutality, but plain that the capture of Westerners was treated as “proof” that CIA agents were stationed round the country’s shores, determined to topple the regime.

In truth, the three young travellers had simply set sail from Malaysia towards Thailand, and it is likely that their boat had been blown off course, perhaps by a tropical storm, and strayed inadvertently into Cambodian waters. It was a time of paranoia in the regime, with great pressure on torturers at interrogation centres – of which S-21 was just one of about 200 – to extract confessions from perceived enemies that would implicate their family members, friends or associates. An additional effort to exterminate Vietnamese people living in Cambodia was in full swing, and the Gulf of Siam was patrolled by navy commander Meas Muth, who would attack and capture vessels with escaping Vietnamese on board.

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Meas Muth is presumed to have attacked Foxy Lady off the island of Koh Tang, which – unbeknown to the sailors – housed a Khmer Rouge military base. Canadian Glass was reportedly shot on sight and drowned. Dewhirst and Hamill were taken ashore at Sihanoukville and then transported overland to S-21. The world then had little idea about the crimes against humanity. Most journalists had either fled or been murdered, the country was closed to visitors, its borders heavily policed, its airports inoperative with the exception of a once-weekly flight to Beijing.

Having been innocently travelling along what was known as the “hippy trail”, when they arrived in Cambodia in the hands of their captors, Dewhirst and Hamill had no idea what lay ahead.

Kerry Hamill’s family only discovered his fate when neighbours rang and told them to get the local newspaper. Rob Hamill, in the documentary Brother Number One (2012), which he also co-produced, describes seeing the front-page news: his brother was presumed tortured to death by the Khmer Rouge, after photojournalists entered the country with the regime’s collapse and found evidence of genocide at S-21.

The family never recovered. Rob’s older brother, who had been particularly close to Kerry and was separated from him in age by just over a year, took his own life months after hearing the details. Their mother, Esther, excused herself every Christmas Day for the rest of her life – “just to go to the shops” – to sit quietly alone at Kerry’s gravestone in New Zealand, though they were never able to recover his remains from Cambodia.

“I did a lot of pretty wacky things [to try to process the grief],” Rob Hamill says. “I took some soil and water from a river and some from the sea near where we grew up, and I took it to Cambodia and had it blessed by a monk at Tuol Sleng, and sprinkled it there.” He poured the rest of the water at the spot where he had been told a Westerner had been set on fire, and describes the process of grief as “beautiful, if done in the right way”. But there have been times when it threatened to destroy him.

When Rob started his successful row across the Atlantic in 1997, he had a breakdown in the middle of the ocean. “I was a mess,” he says. “This was 20 years after the fact, but every day, around the same time, I wept like I’d just heard the news.”

Suddenly, out on the water, away from his family and friends, and far from the shores of New Zealand, Rob got a sense of how Kerry must have felt in the final hours before he was captured by the Khmer Rouge. There is, he says, no closure.

“I’ve not been able to process it and I don’t want to,” says John Dewhirst’s sister, now Hilary Holland. “I didn’t want to feel better – it would make me feel disloyal, in a funny way. It’s the manner of his death – not that I know exactly how he was finally killed – but the days and weeks of torture, the total inhumanity of it. I can’t process that to make it comfortable or right … It doesn’t seem right to be able to put it away in a box … How can it not be terrible today when it was so terrible then? Multiply that by all the millions who were killed by the Khmer Rouge. I don’t even want to sit comfortably with that.”

Rob and Hilary met each other when Rob was researching his brother’s life and death for Brother Number One (“He was on quest,” says Hilary, “but I was destroyed”). In 2009, 30 years after the fall of the Khmer Rouge, yet at a time when none of its senior members had been held to account, Case 001 opened at the Extraordinary Chambers in the Courts of Cambodia. It was a hybrid court, with foreign judges from the UN sitting alongside Cambodian counterparts. The case was against the man who ran S-21 and ordered the murder of thousands.

It was this man – Comrade Duch – who had remembered Dewhirst and described him as “polite” to Irish photojournalist Nic Dunlop when they encountered each other in 1999. Duch had gone into hiding after the collapse of the Khmer Rouge but, according to Dunlop, seemed fairly unconcerned about the idea that he might be prosecuted for his role in the regime. He had converted to Christianity and had put his past behind him.

Dunlop remembers Duch as a man living in the jungle who “knew how to swim with the sharks, knew how to manipulate … He was a classic survivor. The survivors [of these sorts of regimes] are the thieves, crooks, the murderers … I’ve met many people like this and they’re quite ordinary. We don’t like to admit that we all have that potential within us. We are all violent.”

For Case 001, Rob Hamill was invited to travel to Cambodia to give a victim-impact statement, and he asked Hilary to join him. “I couldn’t have done it,” she says. “I wrote something to him before he gave evidence. They were very clear, very strict about not allowing people to say anything on behalf of anyone else, so he incorporated my words into his own statement.”

The email that Hilary sent to Rob back then makes for difficult reading. “It is 31 years since our brothers were murdered,” it reads. “Why does my grief and pain feel undiminished by the passage of time? … When I first heard of my brother’s death and for a long time, I felt that if it was possible to die as a result of emotional pain then I would. I couldn’t see how my heart could continue to pump and my lungs to breathe. The physical pain was so intense and that pain was continuous … The part of my brain that should have developed and retained the memories of John’s life has been taken over by the fact that his humanity was taken from him. That means that there was nothing of him, no soul, no self for me to keep.

“At his death his body was destroyed, before his life ended his humanity was taken – forever. It feels that there is simply nothing of him. Nothing for me to share with my family and nothing for me except almost unendurable pain … You might think that I’m emotionally fragile but I’m not. I am very, very robust – usually the emotional rock for everyone around me. Please take that strength with you to Cambodia. You can make a difference and you will.”

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In video footage of Rob’s courtroom testimony, he sits opposite the small and unassuming Duch, looking directly into his eyes as he addresses him. “At times I have imagined you shackled, starved, whipped and clubbed viciously – viciously,” he says, with emotion, before going on to detail some of the methods of torture used in S-21. “I have imagined your scrotum electrified, being forced to eat your own faeces, being nearly drowned and having your own throat cut … Today in this courtroom I am giving you all the crushing weight of emotion; the anger, the grief and the sorrow.”

Duch promised that, as part of his penance, he would meet with any victims’ families who requested it. Rob put in a request after the trial, but it was turned down. He has never been given an explanation as to why. “What everyone wanted was for [Duch] to break down in tears,” says Dunlop. “He did that very adeptly and who knows if he was serious – does it matter?”

That was one of the most unexpected parts of Duch’s trial: he did shed tears, most bitterly when confronted by a Cambodian man who wanted to know where his wife had been executed, and he did express remorse. Nobody had expected that reaction.

Media, judges and families of victims had been prepared to face an unfeeling psychopath. By the end of the trial, however, even Rob Hamill believed he had seen “moments of sincerity” from Duch. “I didn’t know what to expect or hope from the man who oversaw the torture and murder of my brother,” he says. “He didn’t answer my questions about where I could find Kerry’s remains or where he was killed. And he knew – he knows what happened to those Westerners. They were prize prisoners. At that point, what did he have to fear?”

Andrew Cayley was the international co-prosecutor of the Khmer Rouge Tribunal between 2009 and 2013, with the UN. “It was the toughest job I’ve ever done,” says the Briton, who has also worked on cases investigating serious violations of international humanitarian law in Darfur, Sudan, and Srebrenica, Bosnia and Herzegovina.

Cayley remembers Duch as “an extremely complex character. He was very intelligent … I could never work him out. At the end of the appeal, he said, ‘I want to go home.’ It was like he’d misunderstood the process, or was so mentally damaged himself. He had admitted he’d done all these things, the killings and so on, and now he thought he could just go home.”

In an effort not to be sent to prison, Duch repeatedly used the well-worn and discredited excuse that he had been “just following orders”, may have been killed himself if he did not follow those orders, and was shocked when that was not accepted as a legitimate defence. The pictures at Tuol Sleng Genocide Museum of children, women with toddlers and parents holding newborn babies, photographed before interrogation and extermination … Duch meticulously oversaw their violent deaths.

To this end, Duch employed children: teenagers and young adolescents were responsible for beating or shooting people at the Killing Fields, and torturing confessions out of inmates at interrogation centres. Some of the youngsters were taken from their families, others from orphanages.

“It can be easy to make youngsters cold-blooded,” says Cayley, “to get rid of their humanity and empathy … One of my interpreters while I was in Cambodia had survived torture under the Khmer Rouge and he was terribly scarred. There were big scars around his wrists and ankles from the manacles to prove it. He was very bright but he’d had to leave university [when the Khmer Rouge came to power]. He spoke French because he’d had a good education.

“He was on one of the communal farms he’d had to join, working the fields, and somebody spoke to him in French and he replied in French. That was it. He was arrested and sent off to a prison camp – not S-21, but somewhere similar – and he was being held by one of these groups of young captors, they were aged between 10 and 15 years old. He identified one of the children as the leader and he used to tell this boy Aesop’s fables. And every time there was a movement of prisoners for executions, he would make sure to tell him a tale, and the boy would let him live.”

Because these killers were so young and so numerous, there is little appetite in Cambodia to prosecute. Survivors often say that they bear no ill will towards the children who took part in their torture because they realise they had been brainwashed. Twelve years after the UN-backed Khmer Rouge Tribunal began, only three people have been prosecuted for their part in perpetrating crimes against humanity – Duch, Pol Pot’s right-hand man Nuon Chea (known as Brother Number Two) and Khieu Samphan, the chief of state. Members of the present Cambodian government have said that further prosecutions would “risk civil war”.

One of the stories that Cayley says stayed with him long after he stopped working on the trial in Phnom Penh is that of a woman called Bophana. “She had fallen in love and was engaged, and she and her fiancé were split to different parts of the country when the Khmer Rouge got in,” Cayley says. “They kept in touch by sending love letters, and she was caught, sent to S-21 and tortured and killed. Her crime was actually written down as ‘being in love’. The regime saw love between people as worthless.”

Youk Chhang is executive director of the Documentation Centre of Cambodia and winner of the Ramon Magsaysay Award, which has been described as Asia’s Nobel Prize. He has been bringing together documents relating to the Khmer Rouge since 1995, noting everything he is told by former members of the regime and keeping vast records of confessions and interrogations so that if a family member comes searching, they will be ready and accessible.

He gladly takes donations and funding from universities (the centre was originally founded as part of Yale University’s Cambodian Genocide Programme), but will not take payment for the long hours he spends helping relatives find out about lost loved ones. “I want to do it from the heart,” he says.

Youk Chhang is a survivor of the Killing Fields himself, and lost nearly 60 family members, including his father and five siblings. He runs the documentation project primarily “to take revenge against the Khmer Rouge who made my relatives suffer”, but adds that former members of the regime were the first people he spoke to. “None have refused to speak to me,” he says. “So it was a great help from them.” Their openness, he believes, comes because “they do not want to be [held solely] responsible for the crimes ordered by their superiors”.

As time goes on, Youk Chhang is understandably concerned about the world collectively forgetting the Cambodian genocide. Cambodian journalist Naren Kuch feels the same, admitting that when she was younger, she did not believe in the horrific stories – her mother had refused to talk about them – and the most she had heard about the regime before the establishment of the UN tribunal were radio snippets from her childhood, when “I thought Khmer Rouge meant people with red-colour skin”.

She has since spent most of her career speaking to former Khmer Rouge members. “I felt sometimes sympathy for them after speaking to them, because most them were so young at that time when they were used to commit murders and torture, and many were orphans. For me, I put blame on the former top leaders of Khmer Rouge regime, not former ordinary cadres.”

It was through Naren Kuch that I was able to interview two former S-21 inmates, one of whom – the aforementioned Chum Mey – says he was detained next door to Kerry Hamill and subjected to the same methods of torture. He says he was repeatedly shocked with electricity and had toenails pulled out.

Another survivor, Bou Meng, details how he was arrested by Khmer Rouge who “cheated me by saying that they would take me to train young people to paint artwork at the Royal University of Fine Art. But I and my wife were instead taken to S-21.”

Like John Dewhirst and Kerry Hamill, Bou Meng was instructed to admit to joining the CIA and to produce a detailed account of how he had been recruited, probably because he had been identified as a counter-revolutionary intellectual due to his education. His wife died either during interrogation or not long afterwards. He says he remembers seeing Hamill and Dewhirst during his time at S-21, eating watery porridge with other prisoners, and wearing only their underwear and sleeping “directly on the floor”.

Hilary Holland eventually travelled to Cambodia herself. After visiting Tuol Sleng Genocide Museum, she headed for Youk Chhang’s Documentation Centre of Cambodia, where she was presented with a file of her brother’s confessions: one in handwritten English and identical in content to the typed confession at Tuol Sleng (next to a photograph that, somewhat mysteriously, she says, is not of John, “and neither is it Kerry”; it may be a French or American diplomat whose documents were mixed up with those of another Westerner), and two handwritten confessions in Khmer, which she has never had translated.

What is particularly intriguing is that the “official” confession at Tuol Sleng is dated September 5, 1978, whereas the two sets of Khmer documents are dated September 17 and October 13, 1978. Why did Duch or his young aides deem it necessary to continue recording information about John after his confession had been typed up and marked with his fingerprint? Does this suggest he was kept alive for longer?

Kerry Hamill’s confession is heartbreaking in its humour. Pressed for details about his supposed CIA commanders, the New Zealander gave their names as Colonel Sanders and Captain Pepper (presumably either a modified reference to Dr Pepper or The Beatles’ Sergeant Pepper). Another he gave as Major Ruse. He used his parents’ friends’ names as fellow officers and his home telephone number as his CIA operative number, as well as emphasising the importance of a commander who taught public speaking named “S Tarr” (his mother’s name was Esther, and she was known for her public speaking talents).

Rob Hamill believes Kerry did this to send “a message of love and hope” to his family, though the chances of emerging alive from S-21 were extremely slim.

Reading Dewhirst and Hamill’s typed confessions today gives a vivid impression of what each man was like. Kerry describes his “CIA training” as centring around survival skills, three-week camping trips taken on Great Barrier Island, where “recruits” learned “the arts of spear-fishing, making and using fish traps and animal traps, recognition of edible grubs, roots, fruits and berries, and the proper preparation of these foods”. He writes of constructing shelters and taking another “practical session in survival” in the Southern Alps, on New Zealand’s South Island, where he learned “the techniques of rock- and ice-climbing, how to use rope, that is, knots, splices and slings”, as well as “how to set up and use a flying fox” wingsuit.

Kerry mentions an expedition along the Wanganui river, on New Zealand’s North Island, where he did “boat handling” and “target practice”. The controversies between fictional CIA officers that he invents were actually political scandals in Australia that were widely publicised in the 1970s. One can well imagine, when reading the words Kerry told his Khmer Rouge torturers, the childhood Rob Hamill speaks of, with hikes, exploration, camping trips and adventures on river and sea.

John Dewhirst’s official confession speaks of cultural institutions, detailing the books about socialism and politics that he had recently read, interspersed with references to international current affairs. He writes of educating “communist youths” at the student union in Loughborough, and conjures up a secret CIA building down the road from his college campus, which he says is hidden behind a sign for “Loughborough Town Council Highways Department of Surveyors Office”.

He describes methods of map drawing – something we know he was talented at, since he was Foxy Lady’s navigator – and mentions the name of his sister, Hilary, and the pub she ran. By the end of the “confession”, one has the impression of an erudite, contemplative young man with a love of debate, photography and creative writing.

I expected the two Khmer documents that Hilary sent me from John’s file would be nearly identical to his official confession, and am surprised when my translator, a former reporter for The Cambodia Daily newspaper, called Soumy Phan, sends back 30 pages of information that no journalist, nor any of John’s family, has heard about before.

Soumy has attached a note, which reads, “Having worked through these documents, I find it so interesting. I hadn’t heard about the disappearance of these foreigners before. If this confession is authentic, I would have to say that I learned a lot of new things about the Cambodian genocide … I never thought such documents could exist.”

The first set of documents, dated September 17 (12 days after John’s official confession is dated), comprise 30 pages describing “CIA techniques” around the world. They speak of the Cambodian stand-offs with Vietnam and predict a Vietnamese invasion in 1979 (as it turned out, Vietnam invaded Cambodia at the end of 1978 and toppled the Khmer Rouge a week into the new year). There are long discussions on the situation in Taiwan, how Maoist communism differs from Cambodian communism, why the Soviet Union is gaining ground in Europe, and how the CIA operates internationally in communist countries in the same way that the KGB operates in capitalist countries.

There is information on the Moro National Liberation Front in the Philippines, trade relationships with Thailand, the possibility of the Indian subcontinent providing fertile ground for communist revolution in the future. “I don’t believe these are the words of my brother,” says Hilary, when I show them to her. “The one thing that does jump out, however, is the reference to being given a new assignment on 21 March 1978: my birthday.”

Perhaps John was only asked to confirm theories of Duch or senior Khmer Rouge cadres at this point and, given little freedom to provide meaningful input, inserted a date that he knew would speak to whoever came across the papers in future.

The final document, dated October 13, 1978, contains much more evidence of John. At the top, “American” is scribbled out and replaced with “Anglais”, and his name, originally written as “John Dalson Youferk” is replaced in a different hand with “John Dawson Dewhirst”. It is a lengthened version of the official confession, incorporating some of the notes about the global spread of communism from the September 17 papers. Long paragraphs are dedicated to various political controversies and groups in Japan, as well as Tokyo coffee houses where intellectuals would meet, clearly referencing his time working for The Japan Times.

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He names the best types of cameras to take photographs “for the CIA” and the lighting that would be needed to do it. I recognise the name Michael Lebowitz – supposedly John’s main CIA commander, mentioned right at the top of his longest “confession”. Lebowitz is now a well-respected economist and professor at Simon Fraser University, in Canada. In the late 1970s, he was a fledgling socialist writer and fringe speaker.

I contact Simon Fraser University to ask whether Lebowitz might have travelled at that time and met John Dewhirst in person. In his reply, he tells me that he didn’t travel to Asia in that period, but that he had spoken in the Canadian hometown of Stuart Glass. “It is possible that I may have been an influence on Stuart,” says Lebowitz, “who then shared ideas on long sailing trips.”

Elsewhere in the confession, the nature of Dewhirst and Hamill’s arrest is described: Dewhirst says that he was shot in the left hand and that he and Hamill used Foxy Lady as a shield and “hid in the sea” when they were first approached by Cambodian sailors (Glass having been fatally shot). He mentions “CIA meeting places” that are clearly destinations he visited or had intended to visit: Viengtai Hotel, which still stands off Bangkok’s Khao San Road; a Menlyn Hotel in Seoul; a beach on Koh Samui.

Some of what he confesses to is absurd – at one point, he details being recruited for the CIA as a primary- school-age child by his father in England, and his father taking his salary of £500 a year because he was too young to receive one legally himself. Other sentences seem like they have been inserted by someone else, such as the tales about meeting with nefarious Thai and Vietnamese refugees fleeing Cambodia alongside American CIA agents to plot the downfall of the Khmer Rouge.

“The Khmer Rouge were assiduous about documentation,” Cayley says. “They were very much like the Nazis in that way. We were able to build strong cases against individuals like Nuon Chea, the deputy to Pol Pot, because he implicated himself in the documents we recovered. He was given lists of prisoners at S-21, and he would write on the lists what to do with those prisoners; he would make notes in the margins, often giving instructions to kill.”

The confessions of tortured prisoners are not the only documents that made their way out of S-21. The liberating Vietnamese also found a pamphlet written in French and English called the “Regulation of Security Agents”, which Hilary draws my attention to. This pamphlet was specifically given to foreign prisoners at the interrogation centre; although they were otherwise treated the same as their Cambodian counterparts, they were additionally expected to adhere to its set of rules. Most of the rules are unsurprising (“Don’t try to escape”; “Do sit down quietly and wait for orders”; “If I ask you to do something, you must immediately do it without protesting”) but some are devastating in their cruelty. “The one that lives with me daily,” says Hilary, “is, ‘During the bastino [whipping] or electrisation, you must not cry loudly.’”

John Dewhirst and Kerry Hamill were just two people caught in a genocide that took the lives of more than 2 million, but as I researched their stories, I feel that I came to know fearless, vibrant Kerry, whose childhood had been spent on the idyllic shores of New Zealand, and gentle, creative John, who was born a few streets down from me, in Newcastle. I often woke in the night thinking about the weeks they spent behind bars at S-21.

“I did think about the idea of forgiveness,” says Hilary, “but I felt it was impossible. I keep going back in my head to inhuman acts – the sorts of things that were done to John can’t be forgiven by a human being.”

Rob Hamill tells me he has tried to find solace by researching rituals of grief in cultures across the world. He takes comfort in the messages left for his family in Kerry’s confession: “The sense of loneliness he must have been feeling, and then to write a document of such dignity … It was an inspiring act.”

When I inquire about John’s employment at The Japan Times, just one former employee, now only a sporadic contributor, is left who can recall what happened. Through Toshie Yamashita, I learn that John had boarded a yacht to Hong Kong before he made his way to Malaysia, and that he asked his friend David Kawakami to keep his belongings, because he planned to return to Tokyo a few weeks later “after having some fun”.

I ask Hilary if she knows that some of John’s belongings had been kept there. “Oh yes, they actually sent the belongings back to me,” she says. “It was mainly cameras, photographs.” I say I can imagine the moment of opening the package being very poignant. “Yes. And I ended up using the photographs in an Oxfam campaign. There was a technical college in Phnom Penh which had fallen into disrepair during the regime. Oxfam renovated it and I used the photos to raise money.”

A letter Hilary kept from 1998 details the work done by Oxfam in the region as a result of the donated photographs. It reads, “The appeal fund which was set up in your brother’s name was used to fund student placements for pupils who could not afford to pay tuition fees. The fund was initially intended to pay for 10 student placements. However, the appeal was a big success and raised double the amount expected, so it actually paid for 20.

“These placements were given to young people from very poor families whose prospects for the future would have been very bleak. A place on the training course would provide them with skills which are very much in demand and would almost certainly lead to a job which would supply much-needed income for themselves and their families.”

The last time I speak to Rob Hamill, he is about to set sail with his wife, Rachel, and three children, Finn, Declan and Ivan, to Fiji. They plan to travel around the world together this year, cataloguing their journey on a YouTube channel called “The Cruising Kiwis”, and are considering taking their yacht through Southeast Asia to Koh Tang island, off Cambodia, mirroring Kerry’s journey before being abducted by the Khmer Rouge.

“I see this is a way to acknowledge my brother and the suffering that the people of Cambodia incurred, and to create learnings for my kids and those who follow us,” he writes in an email before the family sets sail.

Rob Hamill also tells me that those who grieve always want to say their relatives should be remembered “so this never happens again”, but that, he adds, “is a lofty goal. Really, all of this is just a personal experience.”

And perhaps it is best to keep it that way, to keep the story personal, because John Dewhirst and Kerry Hamill were not political figures. To the paranoid Khmer Rouge regime, to Comrade Duch and his cadres, they may have been “CIA agents”, prize prisoners, spies from another world, hell bent on eliminating communism. But to the people who really knew them, they were a friend with a box of photographs, and a boyfriend due back for another trip along the hippy trail; a poetic soul who roamed the hills of northern England on weekends, and a fellow yachtsman with a wicked sense of humour; an aspiring novelist who liked to watch the sun rise across the water, and a beloved son, just beyond the horizon, who would surely be home by Christmas. The Independent