He kept a secret diary, written in shorthand, in detention, during which he suffered what he described as “vindictive and intentionally humiliating treatment”. It formed the nucleus of his book, Hostage in Peking, published in 1970.

When the Reuters news agency assigned Grey to be its man in Beijing in March 1967, he was 28 years old and one of only four Western journalists working in the city. Like most Westerners going to the Chinese capital, he travelled via Hong Kong.

“I had been one week in the Crown Colony of Hong Kong – a tiny anachronistic vestige of the British Empire that was to be the indirect cause of two year’s solitary confinement for me in Peking,” Grey writes in Hostage in Peking.

The Hong Kong riots, which started in May 1967, were intimately connected to Mao Zedong’s Cultural Revolution raging across mainland China. When a mass strike at a plastic flower factory in Kowloon resulted in arrests of pro-Mao demonstrators by the city’s police, the Chinese authorities denounced the action as a “bloody fascist outrage”.

Every inflammatory report in the local media about alleged police oppression and brutality in Hong Kong triggered more anti-British hysteria. In June, Chinese state media called for those in Hong Kong to “mobilise and organise themselves to launch a vigorous struggle against the wicked British imperialists”, as violence on the streets of the colony intensified and police attempted to crack down on the instigators.

On the evening of July 11, Hsueh Ping, a 32-year-old reporter from Xinhua (the New China News Agency), was arrested in Wan Chai by Hong Kong police and charged with rioting and unlawful assembly.

The accused journalist gave his address as 5 Sharp Street West, now the site of Xinhua Tower, the headquarters of the Chinese state news agency. Xinhua was at this time the de facto Chinese embassy in Hong Kong, and the police and courts can have been in little doubt that Hsueh Ping was acting in some sort of official capacity. Not surprisingly, the arrest infuriated Beijing and was described as a “fascist outrage”. Grey knew that, as the only British journalist in China, he was highly vulnerable to reprisals.

When, on July 19, Hsueh Ping (named Sit Ping in the English-language press) was given a two-year prison sentence, Grey was promptly invited to the Foreign Ministry in Beijing and politely informed that his press credentials had been withdrawn. He was immediately confined to house arrest until further notice. While anxious about his safety, Grey was relieved that he was not being charged with spying or sent to a Chinese prison.

“I was now clearly a bargaining counter for the release of Hsueh Ping from his Hong Kong cell,” he wrote. However, his situation was about to become much worse, thanks again to events in Hong Kong.

In August, when three newspapers sympathetic to Mao’s cause were closed by the authorities in Hong Kong, it provoked more anger in Beijing.

According to Grey’s account, during the night of August 18, a mob of angry Red Guards broke into his house, surged into his office and frogmarched him downstairs to the courtyard. There, they forced him into the “jet-plane” position, kneeling on the floor with his arms extended straight behind his back, while he was daubed in black paint and had revolutionary posters glued to his body.

As he stared downwards fearing for his life, the hysterical chanting crowd of about 200 Red Guards ransacked his house, painting slogans over the walls. When Grey was permitted to look up it was only to see his beloved brown-and-white pet cat, Ming Ming, hanging by its neck from a washing line suspended from the roof above him. Grey wrote of his “great helpless anger”.

He was then forced to the basement of the house and locked in a tiny room. The windows were boarded up or painted black and the blood of his dead pet cat was daubed onto the sheets he slept in. He spent the next three months in solitary confinement in that room, which he estimated to be 8 foot by 8 foot.

After three months he was moved to a slightly larger adjacent room that measured about 12 foot by 12 foot and where he completed his 806 days of solitary confinement, reading the works of Mao and practising yoga from a book he had bought on his last day in Hong Kong.

Grey described the mental strain as “wheeling and plunging to the depths of depression and back again to bearable levels of optimism”, and claimed that yoga was the single most important factor that enabled him to endure the isolation. One low point was when he was presented with the rent bill for his own one-man prison and subsequently ordered to pay it.

It was not until October 3, 1969, that the last communist newsman, Wong Chak, was freed in Hong Kong, and workmen appeared as if by magic to unnail the boarded-up windows of Grey’s house. Grey heard updates of his own release on BBC Radio Newsreel and was that day summoned to the Foreign Ministry and told that his “freedom of movement is therefore now restored”.

“My feeling was not one of joy or elation or great emotion. It was simple and overwhelming relief that the great span of time alone in confinement, which had sometimes seemed likely to be unending, was finally at an end,” he wrote.

Five days later, he felt well enough to fly back to London, but he refused to travel the standard route via Hong Kong because the city still left him “ill at ease”.

“The colony was the place which had exploded into riots and brought about my imprisonment in my own house,” he wrote, adding that: “I was sure I would not sleep one minute in the colony of four million Chinese.”

Despite his ordeal, Grey said he had no “fierce enmity towards Chinese people” and it did not make him bitter about China.

Grey returned to China in 1988 for the first time in 20 years to make a BBC documentary. The Chinese Foreign Ministry hosted a banquet for him in Beijing and the official who had placed him under house arrest, Chi Min-tsung, sat beside him during a cordial evening. At its conclusion, Grey was formally declared “an old friend of China” – something he said helped provide a sense of closure to his ordeal.

His last published interview was in 2013, with the Eastern Daily Press in England, where he had started his career as a journalist in the early 1960s. Grey revealed that revisiting his diaries for a revised book on his incarceration had opened some old wounds about his days as “the world’s first modern political hostage”.

“Looking at the diaries again I became depressed […] It triggered something in me,” he said.

More than 40 years after his internment, which had been triggered by events in Hong Kong, Grey was formally diagnosed with post-traumatic stress disorder.