WHEN Frank Searle vanished from his caravan on the South bank of Loch Ness there were many rumours about him. His body lay at the bottom of the loch; he had gone off treasure-hunting on the West Coast; he had taken that trip to America and gone on a lecturing tour boasting about his Nessie-sighting exploits; he was metal-detecting in the south of England.

Few among the local community of Nessie hunters seemed sorry to see him go. "He left, " recalls Adrian Shine of the Loch Ness Project, "to the regret of no one." That was 1983 and between then and this year, no one knew what had happened to him; it seemed he had disappeared off the face of the planet. The fans who had once received regular newsletters found their letters met with silence. The man with the camera and the Clark Gable moustache who had once seemed such a fixture of the loch, seemed more elusive than the Loch Ness monster itself.

In the history of the hunt for the Loch Ness monster, Frank Searle is a notorious figure, famed for his blatant fakery. The photographs that he took of the monster during the Seventies were instant classics, reproduced in Scottish tabloids, mainly the Daily Record, yet also among the crudest hoaxes: one looked like a log, another was clearly a rephotographed collage of a cut-out dinosaur from a postcard. Yet another took as its starting point a fence-post sticking out of the loch. One even featured a UFO swooping over the loch. It seemed the newspapers didn't mind the authenticity of what they reproduced - the same paper might run the photographs one day then later debunk them - what mattered was that the Nessie phenomenon sold papers. In the battle for circulation, with a part credulous, part incredulous audience, even a fake scoop was a scoop.

Artist and documentary-maker Andrew Tullis first became interested in Searle a couple of years ago when he was working on an earlier project from Lochnessside. It seemed that whenever Searle's name was mentioned, people wouldn't want to talk. He was considered an unpleasant, at times violent character. There were tales of him possibly having been involved in a petrol-bombing of Loch Ness naturalist Adrian Shine's camp, speculation that Searle was the man who wrote in red graffitied letters on the wall of Urquhart castle 'Shine Con Man', and talk of death threats. Nessie hunters regarded him as having tainted their profession. Tullis was intrigued.

"It fits in with my other interests in Scottish culture. There does seem to be this sort of strand which runs through our history of people running pretty fast and loose with the facts. It's there in the Tartan story, in the Ossian story and in the Nessie story. There is a whole history of creativity around that." What Tullis was interested in was not sighting Nessie, but hauling out of the water Frank Searle, the man behind the lies and fabrications.

Searle, who died in March this year, just a week before Tullis found him, first pitched up at Loch Ness in 1969, one of the busiest years on the loch. The year, notes Tullis, of the moon landings. "People trying to get to the moon and people trying to get the monster, " he muses. He spent 14 years there, a lone stalker, armed with just a camera, living at first alone out of a tent and later in a caravan with a series of assistants and 'girl Fridays'.

There was a romance to the whole story, to the character he invented and the life he lived. Searle once told schoolgirl Joyce Cattanach that he had Professional Monster Hunter written in his passport. He considered himself to be one of the last great white hunters, and even hoped to capture the monster, going out in his boat with shark-hunting style tackle. In his romantic fantasy of himself as colonial-style explorer and adventurer, however, Searle wasn't so different from most of the monster hunters. "Most of them, " says Tullis, "were informed by that kind of end of Empire stuff. It's interesting that these people came up to the Highlands like this last piece of the Empire which no one had ever investigated properly. It's on your doorstep, for lazy adventurers and explorers. You can get in a car and drive up there."

Before Searle there had been others. The history of the Loch Ness monster begins not, as many might think, way back in the mists of time, but in the Thirties, when there was a series of reports of a large creature in the loch. It began with a sighting, later found to be exaggerated and so insignificant it could have been "ducks fighting". Local water-bailiff Alex Campbell, a young man raised on local legends of water-kelpies, is believed to have been responsible for elevating the sighting from fish to monster. It was in the Sixties, however, that the story really took off, when David James, a one-time Colditz escapee, set up the Loch Ness Investigation Bureau and turned the search into an alternative national institution. Searle arrived towards the end of this period.

"Sadly, " says Adrian Shine, a naturalist still researching the loch, "he was everything that the public at large might think investigators are: obsessed, fraudulent. His fakes were the crudest fakes I've seen from Loch Ness. And, of course, he was a highly unpleasant individual as well. I think he had an overweening arrogance. Shame to speak ill of the dead, but he was a most unlikeable rogue."

These days Steve Feltham is the only monster hunter left in the Frank Searle tradition, a solitary watcher, always out there with his camera at hand. When I call him he is sitting in the front of his caravan beside a fire, looking out at the loch. The only sign of life out there is a lone duck. In the 15 years he has been there, he has seen something suspicious only once: a movement shooting across the water for about 10 seconds like a torpedo. In that time too, the frequency of reported sightings has dropped. In the Sixties it had seemed like people were seeing the monster every week, but this year, he says, there hasn't been much: only a couple of photographs.

Feltham was a teenager when he visited Searle's caravan. "He had a gallery, a lean-to shed full of photographs, and my impression looking at them was that he didn't have one genuine photograph. I asked him, 'Have you got any real photographs of it?' That didn't go down well. 'Real photographs, boy? They're all real photographs.' But I could see they weren't. They just looked like things bobbing about, logs mainly."

When he arrived at the loch, Frank Searle was it seems, like Feltham, a true believer. He really believed that he, just by surveying the loch, could be the man to make the big sighting. What turned him from an honest hunter into a fraud and faker? It seems a dispute with the Loch Ness Investigation Bureau is at the heart of this. The bureau had asked him if he would give any photographs he took of the monster and said they would sell them to the press and he would get some money. He duly gave them one, but a year later they were still sitting on it. Searle started to believe they were hanging on to the photograph in order to pass it off as their own. "The grudge, " says Tullis, "is what prompted him to make the first fake. He must have felt the Loch Ness Investigation Bureau were making some money out of the monster and, well, why don't I?"

Though there is a real feeling that Searle did a lot of damage to the Loch Ness project with his fakery, in fact he was just one in a line of fakers. What is distinct about Searle is the degree to which he alienated the rest of the community. Tony Harmsworth, who now runs tour and cruise company www. discoverlochness. com, remembers first meeting Searle in 1976 and finding him to be a genuine and "believable character". It was only later when he decided to set up the exhibition that the feud began to develop between them. Searle would not give his photos to the exhibition, and once it was running, would send people round to visit. They would often demand to see someone, then ask why Searle's material was not on display.

But Harmsworth would show them how clearly faked it was. "Some of them, " he recalls were really angry that they'd been taken in. But I can understand how they might have believed him. He was a very believable character. You hear about people being conned out of millions of pounds. If Frank Searle turned his talents to making money off people he would have been living in the south of France on a yacht." Harmsworth recalls receiving death threats. One, he says, was stuck to his windscreen in Inverness. "It said something like, 'Your time is running out'."

There are plenty of stories that illustrate Searle's streak of violence. In an article written at the time in Oui, a Playboy-style soft porn magazine, journalist Dan Greenburg describes a scene between Nessie hunter Lee Frank and Searle, in which Lee Frank asked if Searle had ever added humps or fins to his photographs. Searle began to shake and pulled out a knife. "Lee says, " wrote Greenburg, "that if Searle's photos are genuine he has nothing to shake about. Searle lunges at Lee, slams him violently into a tree and screams that he's going to throw him into the loch."

Not everyone disliked Searle. He had his helpers, a string of schoolchildren, "fans", and young female assistants who would man his exhibitions while he trawled the loch. He would advertise for these 'girl Fridays', both by a sign with adverts in parts of the country where the unemployment was high. "Hello girls, " says one of these. "Fed up with the establishment? With unemployment and strikes? I am looking for a new girlfriend/girl Friday to join me in the caravan and become fully involved with me and my project."

Lieve Peten was one of these girl Fridays, a 24-year-old Belgian monster huntress who, for a while, shared his caravan and his bed. In Tullis's film, The Man Who Captured Nessie, she describes that time. "I was living with Frank as a mate. There was no romantic involvement, not for him, not for me, but there was a physical involvement. It sounds harsh, perhaps, but that was the Seventies, people experimented. And there was no Aids back then."

One of the enthusiasts who initially bonded with Searle was Alan Jones, who would come up to the loch every year with a group of schoolchildren and initially helped Searle process his photographs. It was Jones, however, who would ultimately feel most betrayed by him. He recalls on reading a section Searle's book, Nessie: Seven Years In Search Of A Monster, the sensation of deja vu, of having read this before; 1400 words had been taken directly from his own school magazine. Jones never spoke to him again.

Searle later wrote another book which was never published. By that time Adrian Shine had established himself at the loch with his sonar investigations and was as keen to bring credibility back to the project as he was to find the monster. He had heard that Searle had a book about to be published, and knowing that he had plagiarised before, and suspecting too that there was much that would be defamatory to him in the book, he set about preventing its publication. Shine is honest about his own desire to drive Searle away from the Loch.

"Basically, " he says, "we wanted to get rid of him, and in effect we did because he went too far. We had removed any remaining credibility that he had by preventing the publication of his book. Hence he had every reason to dislike us. Then he was obviously questioned in relation to this matter of the petrol bomb attack at my camp [Searle denied this]. He had lost a great deal and had gone too far and there was nothing left for him."

We can only speculate why Searle made his fakes. Was it a kind of delusional state he fell into after staring too long at the dark waters of the loch? Was his motivation purely financial? Ricky Gardiner, an art teacher who disputed his photographs, likes to think of them as like paintings of an experience. "I think he was quite a deep character, " he says in Tullis's film, "I think his photographs represented more than just being a fake."

"I can imagine, " says Steve Feltham, "that if he did ever see something he couldn't explain and didn't photograph it, then that would fuel his conviction that there was something out there to be identified. That would keep him watching, and probably in the meantime he thought, well I'll make a few fakes, it'll bring in a bit of money, and eventually I'll get one."

Since Searle's disappearance the Loch Ness investigation has moved on. Times have changed, people are less believing, and it's mostly irreverent tourists who now crowd the loch. Even Adrian Shine's Loch Ness Project now dedicates itself to the science of the loch, and why people see what they see. "You see Loch Ness, " say Tullis, "and it's easy to see why people's imaginations run riot. I don't believe in dinosaurs swimming in Loch Ness. I'm more interested in the cultural story behind Nessie and I think if you investigate that then you get closer to what it might be."

Frank Searle is one big element of that story, an element difficult to pin down. In researching his documentary, Tullis found it difficult to establish hard facts about his life before Loch Ness. "Whenever he wrote about himself, " says Tullis, "he always said he was ex-army. He never really said what he did though. Then he said he was a greengrocer who had quit the rat race to come up to Loch Ness. He painted himself as a sort of East End character, but he wasn't actually born in the East End, he was born in West London." One fact was clearly true: during the war he had lost the lower part of his left leg. "I think, " says Tullis, 'the fact that he lost his leg must have affected his personality. Maybe it drove the more survivalist aspects."

The facts of his life afterwards were even more elusive. No one connected with him, it seemed, had heard from him in the 20 years following his disappearance. Tullis had tried every avenue, and had given up and edited a cut of his film, when he got an answer to an article he had put in a metal-detecting magazine. Someone in Fleetwood, near Blackpool, had seen him. Tullis arrived at the bedsit Searle had lived, only six weeks after his death. All his clothes had already been given away to charity shops and there were few possessions. Little was left associated with Loch Ness: a newsletter and a poster. It was as if that part of his life had been almost, but not quite, edited out. People who knew him in Fleetwood recalled him as a man who kept himself to himself, who talked about the war, not his exploits at Loch Ness.

In his will he left his possessions to his landlord Bill Shirreffs, perhaps his only friend in his last years. According to Shirreffs he talked very little about his time at Loch Ness and there was a feeling that "he had moved on." He seemed a very private person. "He lived very simply, " he says, "loved his garden. Out of all the tenants, he was the only one who tended the garden."

The only photograph there is from this period shows him tending a riotous rainbow of flowers. From 1998, however, following a stroke which confined him to his room, he had been unable to tend the garden. By the time Tullis visited it was an overgrown jungle of weeds.

Searle remains a mystery. A sad, isolated figure, perhaps broken, perhaps contented as he worked on his garden: we will never know. A man perhaps misunderstood. For all the community around Loch Ness seemed to have been happy to see him go, many too, seemed sad to hear of his death. As Steve Feltham says, "I'm disappointed that we were that close to having a final chat with the bloke. To have had the opportunity to sit down and say, 'Admit all these are fakes and why you did it'."

Too late though. A brief glimpse of the monster again, and then he sinks without a trace.

The Man Who Captured Nessie is on Channel 4 at 11.45pm on December 29