Lindesay, 53, has realised his childhood ambition in remarkable fashion. In 1987, he became the only Westerner to make a solo trek on the wall - walking and running some 2,470 kilometres - and chronicled his long march in a 1989 book Alone on the Great Wall.

The journey gave him all the adventure he dreamed of and more. He was arrested nine times, and while being deported met the Chinese woman who would become his wife. He married Wu Qi a year later.

The couple now live with their two sons in Beijing, where Lindesay runs guided trips to less well-known sections of the wall. Over the past 20 years, he has spent more than 1,300 days on the wall.

Lindesay's many explorations brought home just how much rapid tourism and development was affecting the ancient monument, which isn't a single structure but a series of barriers stretching 6,400 kilometres, built over centuries to keep out marauding invaders.

Streams of visitors, domestic and foreign, left mounting piles of rubbish and defaced the stone with graffiti, while peddlers put up makeshift ladders and ticket booths to collect money from tourists.

'I soon realised that garbage was just the tip of the iceberg. There were no laws to protect the wall,' says Lindesay.

It was basically an open-air museum, he says, except there was no curator to guard its treasures.

He began organising clean-up campaigns along the wall, galvanising hundreds of mainlanders into action. In 2002 he founded an NGO, the International Friends of the Great Wall, to help protect the ancient structure from further man-made damage.

To help raise money for preservation work, Lindesay turned to a fellow explorer for help and inspiration: William Geil. An American missionary who traversed the wall a century ago, Geil documented his journey in his travelogue, The Great Wall of China, which featured photographs and detailed field notes.

Lindesay came across a copy of the missionary's book and decided five years ago to revisit the sites pictured to reshoot them for a comparative look.

'Geil carefully photographed his 82-day traverse of the Great Wall in 1908, producing hundreds of glass-plate negatives. From 2004, I took copies of his photographs back to the wall to discover if the locations were still there and, if so, what changes had happened,' he says.

The result is The Great Wall Revisited: From the Jade Gate to the Old Dragon's Head, which Lindesay describes as 'a journey along the wall in the past and the present'.

It highlights changes to the wall throughout the last century through Lindesay's 'rephotography' and vintage images from Geil and other photographers, with some fascinating historical narrative.

Lindesay, who first learned of Geil when gathering information for his first wall trek, had been searching in earnest for information about the missionary since 1999.

'I delved into the Geil family wills, where I discovered that his widow had adopted a little girl, probably her cousin's daughter, who then inherited the Geil estate. I traced her sons, the adoptive grandsons of William Geil, who told me their grandfather's library had been auctioned off in 1960.'

Fate intervened in February 2008 when Lindesay received an e-mail from an historian from Geil's birthplace of Doylestown, Pennsylvania. The historian had been given boxes of Geil's diaries, photos and other travel-related materials by the daughters of a local collector who had died in 2005.

Last summer Lindesay visited Doylestown to view the collection.

'I was amazed. I couldn't believe it,' he says. 'Here were views of the Great Wall I had discovered - many were areas where no other outsider had ever been. They were wonderful photographs and drawings.'

He managed to borrow some items for an exhibition in Beijing last year to mark the centennial of Geil's exploration of the Great Wall. As in Lindesay's recent book, the juxtaposition of dozens of the missionary's photographs with his own images was instructive: some guard towers and sections of the wall Geil visited had been reduced to rubble or disappeared. Geil's photos preserve parts of the wall that no longer exist, Lindesay says.

A keen athlete from an early age, Lindesay often competed in marathons with his two brothers while growing up. It was his brother Nick's proposal in 1984 for them to run from coast to coast along Hadrian's Wall, the Roman-era fortification built across northern England, that reminded him of his childhood goal and that the opening of China made it more than just a pipe dream.

Preparing to visit the mainland, Lindsay spent months struggling to find information about the Great Wall in British libraries, museums and bookshops.

'There was no internet in those days, so I went to local libraries. Even then, there was often no book [on the subject].'

The lack of information only heightened his curiosity. He wrote to map and atlas publishing companies, scoured Britain for relevant information and, in the process, learned of Geil's book.

Lindesay says he found a copy in a London specialist bookshop but it cost ?500 (equivalent to about ?1,000 today; HK$12,500), beyond his means at the time and he abandoned the idea of using it as a reference. 'I questioned what use a book published 80 years previously would be to my trip. I needed to know what China was like now.'

So with little equipment or preparation but considerable youthful impatience, Lindesay set out on the adventure that would lead to his life's mission.

He divided the journey into two segments over spring and autumn, during which he 'trekked, ran, walked and limped' for 78 days across more than 2,700 kilometres of the wall. Sometimes he slept in the open, but more often farmers offered him shelter in their homes. Lindesay's discoveries on the trek amazed and delighted him.

'Like most people outside of China, I had assumed the Great Wall was one continuous construction built during the Qin dynasty 2,000 years ago. I was surprised to find so much more than that. There are more than 20 walls built by different dynasties. And not just walls - there are so many interesting architectural and military elements. There are towers, fortresses, gates, barracks, beacon towers, military roads.

'I discovered that it's hard to follow as it disappears, sometimes for kilometres at a time - there's no need to build a wall if there's a natural feature such as mountains or cliffs you can use as part of the defence plan.'

For his efforts to protect the Great Wall and document its changes since that trek, Lindesay was awarded a Friendship Medal from the State Council of China and an OBE from Queen Elizabeth.

Says Lindesay: 'The Great Wall is a rich cultural tapestry that encompasses not only the varied architectural remains, but also the local people who have inherited tales and legends relating to the wall from older generations. It's a distinct landscape that must be preserved.'