Sitting with Michael Munn, biographer to the stars, in a somewhat bleak pub near his home in Sudbury, Suffolk, I can't get out of my head that classic Pete 'n' Dud sketch in which the flat-capped Peter Cook reveals to Dudley Moore the problems he is having being harassed by the likes of "bloody Greta Garbo". Munn, an affable man of 57, has long been a stalwart of Sudbury amateur dramatics; he still harbours ambitions of moving to Colchester, "just for the buzz". As he sips at a lunchtime half of lager, and tucks into his chicken salad, he is telling me of the time that Ava Gardner wouldn't take no for an answer.

"Ava was a brief but very intense relationship," he suggests, matter-of-factly. "After the first time, we'd meet in the afternoons at hotels in Knightsbridge or wherever." I try to picture the scene. Gardner would have been 45 and Munn 17. He'd not long left school in London, was living with his parents, and had recently been working for British Railways as a trainee in their health and safety department. Ava had moved to London to star opposite James Mason as the Empress of Austria in the film Mayerling. The unlikely pair had met, Munn explains, when he had delivered a package to her – he was by that time apparently working as a messenger boy for a film company – and asked to use her lavatory. "One thing led to another," he explains, with a shrug.

The connection was such that before long Ava had, Munn claims, chosen him as the person to whom she would confide all she knew about her ex-husband, Frank Sinatra, and his vendetta against mafia boss Sam Giancana, which in turn became the inside story of the Kennedys' involvement in the murder of Marilyn Monroe. Thirty years later Munn managed to recall all of this "pillow talk" in vivid detail for his extraordinary book, Sinatra: The Untold Story.

Munn, who admits to being a "bit overawed" by the screen legend's amorous attentions, but "soon got used to it", has no taped record of those conversations, beyond his "good memory for dialogue". Why on earth, I wonder, does he think Ava Gardner, the barefoot contessa, chose him as her lover and confidant, and told him things she told no one else?

Munn ponders that question a moment, while he finishes a mouthful of salad. "Well I have a bit of Italian blood, and she loved that kind of look," he says. "Ava had a thing for bullfighters." As a younger man Munn was once a model for photo features in True Romances magazine.

How did the relationship finish?

"She wanted it to continue," Munn suggests. "But in the end the toyboy thing really got to me. I found a girlfriend my own age. I told Ava that it had to end, she wasn't happy, but there it was." He raises an eyebrow. "I mean: she was getting on a bit…"

Over the course of the next hour or two, in the nearly empty back-room bar with its adverts for quiz nights and barn dances, Munn then cheerfully relates some of the other fabulous coincidences and bizarre adventures he experienced with Hollywood stars in his youth (all of which have led, in recent years, to startling and revealing biographies, published at the metronome rate of one a year).

There was the time he met Steve McQueen in Cornwall in 1970 and joined him as a pillion passenger on a spontaneous four-day off-road motorbike trip, staying in "Devonshire country inns", during which bonding experience McQueen revealed to him, as he had to no one else, his violence toward his first wife, the criminality of his childhood and his premonitions of death (a story which, 40 years on, forms the basis of Steve McQueen: Living on the Edge, recently lucratively serialised in the Sunday Times). There was the time that David Niven, stricken with the motor neurone disease that killed him, called Munn in – "Mike, I've got to see you" – told him to bring his tape recorder, and spoke of his secret love child, his illegitimacy and his failed attempt to shoot himself, as well as hinting at an interest in Mormonism and affairs with Princess Margaret and Grace Kelly (this was the essence of the bestselling The Man Behind the Balloon, also serialised over two weeks in the Sunday Times).

Then there was the occasion Munn bumped into Jimmy Stewart's wife Gloria and took her on a guided tour of London and she subsequently confided to him her husband's violent rages and his secret work for the FBI (Jimmy Stewart: The Truth Behind the Legend); or the day trip to Winchester in his "father's old Morris" when Richard Burton, his passenger, had a roadside epileptic fit and came round being cradled in Munn's arms and subsequently opened up with unprecedented candour about the demons that led him to drink (Richard Burton: Prince of Players); or the time Laurence Olivier once gave him an impromptu acting class and, during a tearful exchange, hinted at how – like Munn, apparently – he had been sexually abused by a priest as a child (this became – along with the revelation that Olivier was an MI6 spy – the headline of Lord Larry: The Secret Life of Laurence Olivier); or the period that John Wayne had treated him "as a son" after the pair had worked together on the film Brannigan (Munn had played the part of "Man in telephone box"), and how this led to Wayne confirming to Munn among many other things the hitherto unknown details of Stalin's plot to have him killed (John Wayne: the Man Behind the Myth)…

Munn explains this string of incredible events in the same manner that the other two occupants of the back room of the pub are discussing the starting prices at Newmarket. One of the odder things about his stories, it seems to me, I say, is that it has taken him so long to write about them. Munn was an often anonymous journalist on film fan magazines in the 1970s, and subsequently put together conventional hack books about movies – The Stories behind the Scenes of the Great Film Epics and The Paranormal Experiences of Movie Greats. It is only in the last eight years – after each of the subjects of his books, and just about all of his quoted interviewees have died – that he has chosen to relate these sensational intimacies. When he had such unique access to the stars, it must have been a torment for Munn to waste his time raiding the cuttings libraries to write formulaic titles like Hollywood Rogues. Why was he so reticent?

"I certainly wasn't secretive if people asked me," Munn says. "When I was in the office, it was like: 'Where are you off to?' And I'd say, 'I'm going to spend the afternoon with Ava Gardner.' Everyone knew..."

The only people who didn't know, it seems, were Munn's family – his elder brother and sister, and his mother – who have been more than a little taken aback by the wild superstar life that he has belatedly revealed. "I was round at my brother's for tea recently," he says, with mild irritation, "and these days it always ends up with the same discussion. They go: 'We didn't know you went on the back of a motorbike with Steve McQueen,' or 'What's all this about Sam Peckinpah?' And I say: 'Well you weren't interested. I was the kid brother. Nobody ever took me seriously. You never asked…'"

He must at the very least have felt it was a charmed life though. Didn't the way that even the most taciturn stars always wanted to take him into their most private confidence seem at all strange to him at the time?

"Not really," he says, "because I was mixing with these people from about the age of 13 when I was still at school."

I take a deep breath, and hear myself saying: "How come?"

"You see I got run over by George Raft's Rolls-Royce in Regent Street one day when I was on my lunch break," Munn says, with a chuckle.

"Really?" I say.

"Really," he says. "Raft [a gangster onscreen and, reputedly, off] got out and bundled me into the car. I had blood pouring out of my nose and before I know it I'm in Raft's apartment off Park Lane. They didn't want police and ambulances: that wasn't George's style. But afterwards he allowed me to go and visit on Sunday afternoon whenever I wanted. I tended to go up there most Sundays – my parents didn't know about it; I never thought to tell them. You never knew who would be there. Three or four people from The Dirty Dozen, Lee Marvin. That's how I first met Frank Sinatra too. I sang some duets with him for a laugh."

Munn pauses, looks me in the eye. "It was only later that I became an honorary member of the Rat Pack…"

I first got interested in Michael Munn's fantastical life when his biography of David Niven came out in 2009. Niven's son, David Niven Jr, was interviewed about it in one of the tabloids and expressed incomprehension and anger at some of the claims that Munn made. Neither he nor his brother, Jamie, who had both been very close to their father, could believe what they were reading. They couldn't see how Niven would have been capable, for example, of getting down on his knees to pray, as Munn (who had been a Mormon elder at the time) claimed, not only because of his avowed atheism, but also because of his advanced illness. "If my father wanted to confess something," Niven Jr said, "why would it be to this man? He had lots of friends who were priests; he knew cardinals, for heaven's sake. And why, if Michael Munn was such a good friend, did he never introduce him to us? I also can't imagine what my father was doing in London in 1982 when this interview is said to have happened, because he spent his summers in the south of France and the winters in Switzerland. As for what my father is said to have revealed to this author, well, everyone featured in these stories is rather conveniently dead, so we can't ask them to verify it."

Private Eye ran something about the story, and when Munn's Steve McQueen book came out earlier this year, similar doubts were cast on its authenticity by McQueen's first wife Niele, who believes she would have known, for example, if the actor had been systematically kept in solitary confinement as a young boy in reform school (as Munn claimed), not least because she herself had been in an internment camp during the war, and they had talked extensively about their difficult childhoods. Munn's account of the motorcycle trip was also undermined by McQueen's business partner Robert Relyea, executive producer on Le Mans, which McQueen was in the middle of filming when Munn claims to have met him in Cornwall. Relyea believes that the only time McQueen left the set in that time was when he went with Neile to Morocco for a few days.

With some of this in mind I spent a couple of days in the library reading through Munn's extensive backlist. The recent books, full of first-person revelation, have a far different flavour to earlier stolidly researched biographies of the likes of Gregory Peck and Sharon Stone, once Munn's stock in trade. Not only are they infinitely more entertaining (not least because you are not sure which rabbit is going to be pulled out of which hat next); an uncanny pattern emerges between them. This, for example, is how Munn establishes his rapport with Burton (whom he meets while working as an extra):

"What do your friends call you?" Burton asked.

I told him it was Mick...

He said: "Then you shall be Mick to me and I shall be Dick to you. And if Elizabeth [Taylor] comes over then we shall be Mick and Liz and Dick."

He was simply enjoying a tiny word game, but whenever I met him after that, usually as an extra in one of his films, I always said to him "Hello, Dick" and the assistant director would have apoplexy…'

And this is how he becomes intimate with Olivier, whose dressing room he talks his way into (aged 19) while the actor is making Sleuth:

Everyone called him Larry. I felt I couldn't.

"You shall call me Lord Larry," he said. [Thereafter] I would call him Lord Larry and we'd see the faces of people around us look on with horror… He called me "M and M" in such a way as it came out "Eminem" long before the rap artist Marshall Mathers thought of it…

In addition to the uncanny revelations from the stars' own mouths (after their riddling monikers have been established), Munn generally gained a corroborating source (now sadly deceased) of someone whose intimacy could not be trumped. The pattern was set by Ava Gardner, but went on to include Niven's famously difficult wife Hjördis, Jimmy Stewart's wife Gloria, James Coburn (McQueen's great friend), who Munn first met at George Raft's house, and (for Olivier) John Gielgud, who opened up to Munn after he sat next to him on the 73 bus in Oxford Street.

The books tend to round off with a sentimental farewell and a promise from Munn to set the record straight. With Olivier, it went like this:

At the end of my allotted time with him, and after the emotional experience we had shared, he took my hands in his, very gingerly, and said through tears: "Please come and see me again Eminem."

"I will Lord Larry," I replied. I left certain that I would never see him again. This time I was right.

With Burton:

We had our goodbyes. I said "See ya, Dick."

"Be seeing you Mick,"

I had the feeling that I would never be seeing him again. I was right…

When the Niven sons first raised doubts about Munn's account he was asked to provide his tapes as proof of Niven's deathbed confessions, but unfortunately they no longer existed as they had been "chewed up in his machine long ago". In most cases his extensive dialogue did not get recorded in the first place.

"I often," he says, "found myself in a position to discover more about the real lives of stars when my trusty tape recorder was off…" What notes he made are on "scrappy bits of paper" whose relevance only he can understand.

Given the sketchiness of this source material, does he understand why family members might not always believe his posthumous rewriting of their loved one's private life?

"Well," he says. "Does your family know all your friends? To a certain extent the books are me interviewing me. If people don't want to believe it, I'm not in the business of trying to persuade anyone. I just write it exactly as I remember it."

Munn's books have been widely reviewed and variously praised for the "revelatory", "debunking" and "explosive" insights they offer. His rewrites of film history have, of course, been meticulously recorded as gospel in Wikipedia. Munn's publisher through most of his career has been Jeremy Robson of JR Books. Robson claims to have seen some reference material "at the beginning" but mostly, he suggests, "you have to take him at his word to some extent. I believe in Michael's integrity… You can't libel the dead, of course, and I don't believe that Michael does. He's a very moral person…"

When he looks back, Munn tells me, his abrupt shift in biographical style was brought about by two things. The first came after an "odd time in his life" around the millennium. In 2000 Munn experienced his finest hour as a director when he put on Antony and Cleopatra at Sudbury's Quay theatre, taking the part of Antony himself (an ambition he'd pursued "at Olivier's insistence"): "We pulled it off in period costume," he recalls. "My partner Jane made the breastplates from papier-mâché and all that." The production was, however, very stressful and it exposed some of the tensions that can surface in provincial theatre. Not long afterwards Munn took his talents to neighbouring Boxford. As a writer, too, Munn's career seemed to be faltering. He had parted company with Robson Books a few years previously and, a divorced father of three, wasn't sure where to go next.

"If I'm honest," Munn says, "I wrote the Sinatra book for my own therapy. Just straight through. It was only when I finished that I thought: I wonder if Jeremy Robson would like this. The odd thing was that in the book I denied even knowing Frank Sinatra; I didn't need to, because I got such a lot from Ava. The fact that I had known Sinatra since I was 13 was getting into territory I didn't necessarily want to get into…"

The second turning point in Munn's literary transformation was a mild heart attack that he suffered in 2005. After that, he tells me, he thought: "Sod this, I had a good time and I enjoyed being with these stars, so the books became memoirs doubling as biographies. My publisher liked them. And they started getting serialised in serious papers like the Sunday Times." The effect of this has been cathartic on Munn, who seems to have grown into the character of celebrity confessor. He has a lively blog, he does rounds of radio interviews and crops up on documentaries. "I was always full of regrets because I didn't become a director," he says. "I mean I have directed some plays, nothing in the West End. But I had all these regrets and after the heart attack I thought, you know, why not instead start celebrating my life? I thought: I will write what I please."

In some ways, Munn writing what he pleases, his decision to come clean about his alternative life with the stars, fulfils a very contemporary need. As any casual browser in the biography section of a bookshop will quickly realise, it is not enough these days for the writers of biographies to stand at one remove from their subjects; readers and publishers demand more of a connection – a lover, a prodigal son, an ex-wife. Munn takes that to its logical conclusion: he has intimate insights into not one star, but many.

"My publisher Jeremy knows what I can do, and I know what he wants from me," Munn says, "though the time might come that I won't be writing books and I'll get a job at Tesco stacking shelves." So far, however, Munn's Zelig-like appearance in the lives of those he writes about has had a kind of mesmerising effect, not least on reviewers and readers. "I get so many letters from people who say it's great to see it through your eyes because you knew the people so well," he says.

The more Munn talks, the easier it is easy to see why such compliments might be music to his ears. In the pivotal scene of his Olivier biography, he and the actor have a tearful exchange about the effects of alleged abuse in their childhood. "Lord Larry dealt with it by doing what he did so well already – by becoming someone else… I dealt with it by imagining I was some of the great heroes of the screen, such as John Wayne or Charlton Heston."

Heston in particular has been something of a touchstone in Munn's life (a relationship which culminated in the pair collaborating "actor/director to actor/director" over the staging of Antony and Cleopatra at Sudbury, just before Heston "was taken by Alzheimer's"). It began, though, in a more conventional screen idol to fan kind of way.

When he was eight Munn was given a programme of Ben-Hur by a friend who had seen the epic. "I treasured this thing," he recalls. "To me it looked completely real. I had to go and see it for myself, see how the magic was done. So I badgered my parents, and when the film came to our local cinema I just stood there and cried until they took me to see it."

The film was such a hit, he says, that there were no half-price concessions for children; his dad, who "worked at this and that", made a bit of a fuss about having to stump up full price for the three of them. In later life, Munn says, he repaid his father the one and six extra it had cost, because the effect of the film had been so profound. Munn knew there and then he wanted to go into the movies. Though he started writing plays, his school had no drama department; when he came to leave, with no qualifications, he sat down with the careers officer and told him he wanted to be a film director. "He said there is no way to become a film director."

Having started out with British Rail, Munn says, he got into film publicity, and then worked for the now defunct fan magazines Photoplay and Film Review. He would try to get directing tips, he recalls, from the greats – John Huston, Sam Peckinpah – as well as little jobs on sets. His finest hour as an extra was "playing one of the rebels" in Star Wars, though he is not sure if his scene made the final cut. Other than that, of course, there was a lot of hanging around, out in the cold.

Along the way Munn made numerous efforts to write novels, but he could never make them sound credible. The book he would really like to write, of course, is a proper personal memoir, one that puts his life at centre stage. He did a draft, but his agent said it wouldn't sell because no one knew who he was. "You had to be a celebrity of some kind," he suggests, ruefully. Then, happily, he realised that he was a celebrity, it was just that it had taken him half a lifetime to own up to it.

You would imagine that, at his impressive current rate, Munn's production line of revelation would at some point come to a halt. Even he, I suggest, must be running out of chance encounters by now?

"Not at all," he says, cheerfully. "In fact it is hard to know which one to go to next."

Munn is thinking of putting something together about Peter Sellers. He first got to know the actor, of course, at George Raft's house one Sunday afternoon in the school holidays. Subsequently, in a scene not unfamiliar to fans of Pink Panther movies, he "got in a lift with him when I was a publicist, and we had this hysterical lift ride when I was pressing all the wrong buttons". Firm friends after that bonding experience, there were meetings at the Dorchester, and trips to Vienna and Paris. "He opened up to me," Munn says, "about his madness in particular in ways that I have not seen in the other biographies. When I got into my Mormon phase, Peter was obsessed with talking to me about the afterlife."

Did Munn, I wonder, by any remote chance, also become the intimate of Lynne Frederick, the great love and torment of Sellers's later life, who died in 1994?

Funny to relate, Munn says, but he did. In fact: "Lynn wanted me to marry her, when my marriage broke up, back in the early 90s."

Wasn't he tempted?

Munn takes a deep sigh. "I just thought: 'No, I can't do this.' I regret it in a way, because if I had married her she might not have got so involved in drink and drugs... But then she was already down that road, and to be honest, I could live without being a millionaire."

Munn has by this time long finished his chicken salad and declines my offer of another half of lager. While we wait for the bill I mention a couple of the Hollywood stars I've interviewed over the years, Jack Nicholson, Kiefer Sutherland. The problem with interviewing actors, I suggest, is that you can never quite tell when they are spinning you a yarn.

Munn shakes his head. He couldn't disagree more. "If there's one thing I've always been good at it's this," he tells me, with conviction. "It's this sixth sense I have. I can always see right through a tall story."