Perched on a makeshift throne fashioned out of an old mattress, the thin, vulpine figure of Girodias declared that he had found a solution to their financial problems. At this, everyone perked up. They should, he went on, forget about the life of the mind - for the time being anyway - and concentrate on less lofty matters. Much less lofty matters. In fact, they should all start writing pornography for him.

This proposal was greeted with some surprise, but also considerable enthusiasm. So was born the Traveller's Companion series, one of the oddest ventures in publishing history. For the next few years this group of literati toiled away in the mines of erotica - the books they produced being eagerly snapped up by British and American servicemen stationed all over Europe. In Nissen huts from Deauville to Dusseldorf, furtive men in khaki thrilled to their Traveller's Companions.

But then the world moved on, armies went home, tastes changed - and that was apparently that. However, in the past couple of years something quite unexpected has happened. After decades of neglect, the Traveller's Companions have swept back into fashion. Prices for first editions have soared: the full set of over 100 books would now set you back pounds 10,000. Alexander Trocchi, the leader of the Merlin pack, is now regularly cited as a key influence by a new generation of writers, including Irvine Welsh, the author of Trainspotting. And this autumn Young Adam, a film based on Trocchi's best-known novel, will be released, starring Ewan McGregor.

All this has come as heartening news - at least to devotees such as, well, me. It must have been sometime in the mid-1960s that a much-thumbed and pirated edition of one of the Traveller's Companions, The Loins of Amun by Marcus van Heller, found its way to the prep school on the South Coast where I'd been sent to improve my mind.

I can clearly remember the incredulity I felt upon reading it, turning the pages with trembling fingers. What monster of depravity could have dreamt of such things? Who was this brute van Heller? And what on earth was a "pulsating triangle of junction"? It was an experience that could be said to have shaped - and scuppered - much of my subsequent life.

Recently - and now teetering on the cusp of palsied lechery myself - I set out to try to discover what had happened to the Traveller's Companions writers. Some, predictably enough, had crashed and burned; died of heroin addiction, or booze, or simply old age. But others turned out to be very much with us.

Among them is Iris Owens, who, aged 20, arrived in Paris from America in the early 1950s full of grand literary ambitions. It wasn't long, though, before hunger had set in and she found herself answering Girodias's call. No guidelines were given to aspirant authors, beyond a general warning to steer clear of necrophilia. As for the rest, it was up to them.

"I'd never read a dirty book in my life," Owens recalls. "Let alone thought of writing one. But I remember feeling very challenged by the idea. I considered myself a writer and I thought, well, why not give it a try?"

A month later she took her manuscript to Olympia Press, Girodias's publishing company, and waited nervously while he read what she'd written:

Madame de Saint Ange licked the pink stiff blood clotting the tips of Eugenie's teats, breathing, "They're lovely. You need envy no woman. Ah, they're delicious. Do you feel the heat going down to your innocent belly? Do you feel your body is going to open and let all the blood rush out?"

"Yes," the girl giggled weakly.

Girodias was enormously impressed and told Owens that henceforth she would be known by the pseudonym "Harriet Daimler". In his catalogue of forthcoming publications, he noted gravely: "Harriet Daimler struggles against her impossible tendency to write more explicitly than the courts will tolerate."

Then there was the poet Christopher Logue, described by Girodias as "pale, ill-fed and ill-garbed". Logue's first dirty book, Lust, was written under the pseudonym Count Palmiro Vicarion. Logue now reckons Lust is pretty ghastly stuff, but Girodias didn't think so, insisting that the novel demonstrated both "the author's artistic integrity and his deep knowledge of the human heart".

However, the dominant figure among the young pornographers was undoubtedly Alexander Trocchi - the Scottish son of an Italian hotel pianist - who had set up Merlin with money from his girlfriend. Trocchi, slightly older than the others, was a fully-fledged Bohemian - very keen on philandering, as well as ingesting copious quantities of drugs.

"His face," as one of his colleagues recalled, "had something of a Neanderthal cast: a narrow brow jutting from beneath short, brown tousled hair, a prominent hooked nose and the lower part of the face dropping away, with a lantern jaw beneath the mobile mouth. I guessed his age to be around 40. He was, in fact, not yet 28."

Iris Owens recalled that Trocchi looked "like something off Mount Rushmore. He was very exotic - he saw himself as being one of a long line of literary expatriates like Joyce and Hemingway."

But for the time being, all Trocchi's literary ambitions were put on hold. Under the pseudonym "Frances Lengel" he rapidly knocked out his first novel, Helen and Desire, in his room in Montparnasse. Then - just two weeks later - came its successor, The Carnal Days of Helen Seferi. Like everyone else, he was paid approximately pounds 200 per book ( pounds 2,800 in today's money).

Then there was the insatiable Marcus van Heller. Not only was van Heller the most prolific of the Traveller's Companion writers, he was also the most

mysterious. As John de St Jorre puts it in his history of the Olympia Press, The Good Ship Venus, "In the netherworld of erotica, the name Marcus van Heller approaches the stature of legend." Van Heller's first book, Rape - in essence, the happy recollections of an unrepentant rapist - announced him as a very tough customer indeed. Even Girodias was taken aback, telling van Heller it was "almost too brutal". None the less, he went ahead and published it anyway.

This was comparative juvenilia in the van Heller canon. More - a great deal more - was to follow. In 1955 The Loins of Amun appeared, signalling the fruition of van Heller's mature style: a winning combination of ancient history and permanently engorged appetites:

"Oh, don't fight over it," the girl begged, her voice broken with passion. "If one of you can't wait, you can have me together." The men looked at her in surprise. The bulges in their tunics were enormous.

No wonder. Nothing, it seemed, was too strong for van Heller. While other writers may have found themselves held back by a few tattered remnants of decency, van Heller romped right through them. "His thick, powerful legs dug into her thighs as, teeth bared, he thundered into her." These, incidentally, aren't just any old powerful legs we're talking about - they belong to a baboon.

By the mid-1950s the Traveller's Companions were in full swing - the books being published in sombre green covers so as not to attract the attention of the French police. With success came the establishment of a peculiar literary salon that met in the Cafe Tourneau in the sixth arrondissement.

"We all used to write in isolation, but we were a pretty social lot," says Iris Owens. "We'd sit round and drink and read from our work. I don't think any of us felt embarrassed by what we were doing. In fact, if anything, I think we felt quite proud. We were all very keen on the idea of sexual experimentation, both on and off the page. Alex Trocchi, in particular, was quite naughty."

Trocchi was now working on Young Adam, an existential thriller about a work-shy barge operator who finds a body in the Glasgow-Edinburgh canal. The book was to be rejected by a number of other publishers, but Girodias agreed to publish it on condition that Trocchi insert several more sex scenes into the narrative.

Girodias's little scheme had been a greater success than he could possible have dreamed of. Nothing - or almost nothing - could dent his composure. The only occasion he was lost for words was when one of his woman authors phoned to say that she would be late delivering her manuscript because her clitoris had been bitten off in the night.

Occasionally the Olympia Press offices would be raided by the police and books confiscated, but for most of the time the French authorities were happy to turn a blind eye to what was going on. Then, in 1955, Girodias published the first edition of Lolita - Vladimir Nabokov having been unable to find a publisher in America. Appalled by the subsequent publicity, the French Government banned the book. Girodias counter-attacked by suing the Ministry of the Interior - something that had never happened before - and eventually the Government backed down.

And all the while Marcus van Heller kept typing away, books tumbling from his typewriter with hectic abandon. Roman Orgy. The Borgias. The Wantons. With Open Mouth. Cruel Lips. There was, it seemed, no end either to his stamina or to the fertility of his imagination.

Yet slowly the idyll fell apart. Financial squabbles broke out, with the authors believing - with reason - that Girodias was coining it in at their expense. Trocchi went to America where he toppled into heroin addiction. Iris Owens also called it a day and returned home to America.

By 1961, even van Heller had had enough. He wrote one final novel, Kidnap - "he moved towards her, thrusting forward his hips, his pr--- like the searching antenna of some huge and mighty insect" - and then vanished, leaving one giant question mark behind: just who was Marcus van Heller? And what mysteries did his pseudonym conceal?

At this point the story switches in both time and place: to the present day; and to surroundings of impeccable suburban gentility in Chiswick, west London. Among these neatly clipped gardens there is one that is a lot more overgrown than its neighbours.

I have found the house following a tip-off: I go through a somewhat rotted wooden gate, walk up the path and knock on the door. From all around comes the buzz of lawnmowers and the squeak of chamois leather on freshly washed BMWs.

The door is opened by a man who must be in his early 70s but who could pass for 20 years younger. Behind him is a darkened hallway from which comes a strong smell of dog.

"Excuse me, but are you by any chance Marcus van Heller?" I ask.

A shy smile spreads itself across the man's features. "Ah," he says. "I was. Once."

It was in early 1954 that a young man called John Stevenson, then aged 23, arrived in Paris from England with vague ambitions to be a plongeur (dishwasher) like his hero George Orwell. This proved harder than expected, so instead he became the marketing manager for Merlin magazine. The job was a lot less glamorous than it sounded: mainly it entailed hawking copies of the magazine around tables in cafes.

Then came the suggestion from Alexander Trocchi that Stevenson might like to supplement his meagre income by writing pornography. And so - from such deeply unpromising beginnings - "Marcus van Heller" sprang into life.

As Stevenson freely admits, nothing in his life so far had suggested he had either the experience or temperament to be a master pornographer. "I wasn't a natural Bohemian - far from it. Previously, my only experience of writing had been doing a couple of features for West Country magazines. I knew that writing porn wasn't going to be anything I could brag about - my parents never knew anything about it - but I just thought I'd see what happened."

His concern was that his imagination would flag. "I was worried at first that it might be very difficult to retain the erotic fire to write a whole book. Actually I found it remarkably easy. A lot of the writing was very formulaic, of course - you knew you had to have a sex scene of some sort every few pages - but after a while I took to padding the books out with bits of historical research."

For The Loins of Amun Stevenson scoured the British Council Library in Paris, diligently researching the design of Hittite weaponry. None the less, he was surprised to see the book described in Girodias's catalogue as "based on years of historical and archaeological research by the well-known author who is also a renowned scholar in these fields."

"Then after that - let me think - yes, then I did Roman Orgy. That was about the Spartacus Rebellion."

How did you manage to get sex into the Spartacus Rebellion? I ask him.

"Well," says Stevenson. "They went on the rampage a lot."

But however easy he found it to crank out the books, the writing did soon lose its appeal. "You have no idea what it's like writing one sex scene after another," he says in a heartfelt voice. "Trying all the time for new permutations. Believe me, it can really get you down. Occasionally I found it erotic, when I was really getting into a scene. Most of the time, though, I just thought about the money."

At the beginning of the 1960s Stevenson returned to England. As he explains in his as yet unpublished autobiography (provisionally entitled The Marcus van Heller Legend), relations with Olympia Press deteriorated sharply after Girodias took exception to a sentence Stevenson had written in one of the late van Hellers: "Her fingers were sticky with lubrication from his passion." This, claimed Girodias, betrayed a lack of effort: "I think you should see girls more often."

There was worse to come. Stevenson learned that Girodias had commissioned other writers to churn out Marcus van Hellers - apparently believing he owned the rights to the name. "At the time I was amazed to be making any money at all from writing my books," he says. "It was only afterwards I realised that I'd lost hundreds of thousands of pounds' worth of royalties as a result of not having a proper contract."

Stevenson left Paris and, after a spell loading Crunchie bars onto a conveyor belt at a chocolate factory near Bristol, took a job as a press officer for the Central Office of Information. Here was a truly shocking turn of events: Marcus van Heller had fallen into the clutches of the British Civil Service. Respectability, it seemed, had triumphed. The Loins of Amun were nothing more than a distant rumble on the horizon.

The human spirit, however - along with other, more corporeal bits - is not so easily kept down. Slowly, timidly at first, The Traveller's Companions began to enter the second-hand book market - and to shoot up in price. According to Sven Becker of Simon Finch Rare Books, "they have become very collectable. Pornography generally is becoming more fashionable and these books have a real cachet to them. You could probably pick up a good van Heller for around pounds 70. All sorts of people collect them, from postmen to rock and roll musicians."

Recently, all of Alexander Trocchi's books have been republished - including Helen and Desire. But Trocchi, alas, is no longer around to enjoy his revival. After spells in prison and in the gutter, he died in 1984. In Trocchi's final years he and Stevenson, once close friends, saw little of one another. "I'd go to see him occasionally, but it was very difficult because all these people would be laid out in a stupor and Alex would constantly be trying to get money off you."

Trocchi always dreamed of having his work filmed - and now, finally, his ambition has been realised. David Mackenzie, director of Young Adam, says that what he responded to most of all was the "dry, existential sensuality in Trocchi's work. But when I first started thinking of making a film of Young Adam, it was very difficult to get hold of a copy. I remember I had one very old battered edition which I had to Xerox and send round to people.

"Now, of course, it's all over the place. I think it's a novel that has stood the test of time very well. Elements may be dated - Trocchi's attitude to women is somewhat pre-feminist, to put it mildly - but the writing has such energy to it and such a distinctive tone."

In the mid-1960s Maurice Girodias moved to America, where he published various pornographic novels - including one called Mama Liz Tastes Flesh - and a slightly out-of-character illustrated volume on Muhammad Ali.

However, he was dogged by financial troubles and returned to Paris in the late 1980s to an apartment overlooking the Pere Lachaise cemetery. Evicted by bailiffs, he was kept afloat by his brother, Eric, and died of a heart attack in 1990.

Iris Owens, however, is living in New York City and still writing - now under her own name. "I hadn't looked at any of my Harriet Daimler books in years," she says. "But I dug one out the other day and I was pretty impressed by the quality and the vitality. I certainly couldn't have written it now. It made me think about what my mindset was at the time. Although I was one of the few women writers working for Girodias, I wasn't aware of gender issues, or anything like that. It didn't upset me at all how brutal a lot of the books were; how all these women were being abused in chains and turned into passive objects of desire. I'm afraid that struck me as perfectly normal."

As for John Stevenson, sitting in his sparsely furnished living-room with a thin curtain pulled over the window, he insists that he's quite happy to remain concealed behind the rampant silhouette of Marcus van Heller, his double life known only to a few old friends. "Sometimes I do look back and think, God, did I really write that? It's an odd feeling. But it was a very vivid period of my life. A wonderful period in many respects. And to be honest, nothing since has quite come up to it."