Richard E Grant's heroic performance in the cult movie Withnail & I is legendary, but the real Withnail is little known.

It's the end of an era. Withnail and Marwood, the two out-of-work actors whose vibrant, drink-fuelled mischief filled the movie, are at a crossroads. Marwood (the "I" character played by Paul McGann) has a job. Withnail, for all his charisma, has nothing.

Bruce Robinson, who wrote and directed the film, has always said it is autobiographical. He is the "I", leaving to take the lead in the First World War play Journey's End, the start of a career that would make him a director and Oscar-nominated screenwriter. And the man he left behind was his best friend, Vivian McKerrell.

McKerrell, who died in 1995, appears as a note in the margins of movie history: the unsuccessful actor who inspired the cult hero. Now, for the first time, Kate Stacey Lister, who was his partner for 14 years, has decided to speak about the real man behind the role.

She talks of a magnetic person with film-star looks ("a bit like Sean Bean, or a young Richard Chamberlain") who did not fulfil his potential, largely due to a lifelong battle with alcoholism, and died young from throat cancer. "Richard E Grant played Vivian fantastically well, though he was not nearly as beautiful," she says. "He captured the essence of Vivian, how he walked, how he moved, but Viv was mind- blowingly beautiful."

She shows me a photograph she took of McKerrell with Bruce Robinson on the steps of her house, an old vicarage in Nottinghamshire. Robinson is trying on McKerrell's coat, a beautiful tailored tweed, a style later copied for Richard E Grant in the film. McKerrell is sitting on the steps watching, a drink in his hand, faintly amused. The movie won't be made for another 15 years, but they are unmistakably Withnail and I.

"It captures something between them," says Lister, staring at the image in her cosy Edinburgh sitting room as if it is transporting her back more than 30 years. "Vivian and Bruce had the same sense of humour. Being with them all the time was amazingly good fun." She shrugs, wryly: "I left that coat in a cupboard in the vicarage when I moved." The one made for the film was later bought at auction by Chris Evans for 5,000.

McKerrell, who grew up in an aristocratic family originally from the island of Islay, met Robinson at the Central School of Speech and Drama in London, and they became the kernel of a vibrant group of actors, musicians and writers.

Lister, a portrait painter who trained for a time at Glasgow School of Art, met him at a party when she was 28 and he was 23. "I thought he was a cross between a prince and an Irish thug," she smiles. "He was tall, elegant, perfect material for a movie star. He had done some prestigious stage and television work and it seemed like his artistic goals were set fair."

He and Robinson were living in a flat in Albert Street, Camden Town, which would later be immortalised in the movie as a fleapit with a sink full of festering dishes. "It really wasn't as bad as the film made out," says Lister, "though maybe they cleaned it up for visitors coming! It was a real meeting place for all the actors, a very interesting crowd.

"They were a very handsome bunch, the Jude Law gang of their day, they were always in the smart magazines. Someone discovered the original costumes worn by Nijinsky in Ballets Russes in some forgotten trunk, and Vivian modelled them for Vogue because he was so like a dancer.

"That said, they were always broke, and almost always drinking. As soon as I arrived, they'd relieve me of my pocket money for drink, and when that ran out they'd push me through the doors of pawnbrokers with some beautiful family heirloom to get more money – they said I'd get more because I'm a woman."

Many of the antics of the film were taken directly from life, she says. McKerrell did once drink lighter fuel as a substitute for alcohol, and the pair did smother themselves in Deep Heat in an effort to keep warm. "And when friends came round they would hold a hairdryer in their face to heat them up. And they were constantly in front of the mirror doing their hair, which was magnificent to watch because they were so good-looking."

They also had a style-conscious cannabis dealer, not unlike Danny (Ralph Brown) in the movie. "Slade had started this craze for platform boots. This drug dealer has got stuck miles from home in the middle of the night and had to walk home in these platform shoes. He had to stay in bed for a week afterwards. That used to make us cry with laughter."

Whether there was a trip to Cumbria in a clapped-out Jaguar, whether they roamed the hillsides with plastic bags on their feet or ordered "the finest wines available to humanity" in a tea room in Penrith is moot. Withnail and I contains an important kernel of truth: times were changing, careers were taking off, the Camden Town gang were going their separate ways.

But Lister is keen to point out that McKerrell's life, unlike Withnail's, was far from over. "You get that sense at the end of the film – is he just going to disappear into a ditch drunk? But in fact he came to live with me at the vicarage. A new part of his life started which was no less interesting than the film. We had adventures and travel – there could have been another film about all the adventures we had."

In Lister's photographs, she and McKerrell are a striking couple, she blonde and beautiful, he dark and elegant with pale skin, chiselled features and grey-blue eyes (which he liked to darken with prussian blue contact lenses). He loved silk shirts, she says, and vintage tweeds. They listened to Elvis, Miles Davis, James Brown. Their song was Heroes by David Bowie.

Fashion designer Paul Smith was another friend. "He loved Vivian's style. He always made sure Viv had loads of his clothes because he wanted him to wear them. I think Viv was a muse to him, he was so elegant."

So many famous friends visited the vicarage that it was dubbed "the actor's retreat". Bruce Robinson and his partner, the actress Lesley-Anne Down, were frequent visitors. Peter O'Toole, Marianne Faithfull (with whom Vivian appeared in the 1974 film Ghost Story) and Oscar-winning designer Peter Young were among the guests at Lister's regular parties. She painted portraits of many of the famous faces of the day, including Charlotte Rampling and Julie Christie, and, of course, Vivian.

McKerrell took to country life so naturally that the dearth of acting jobs ceased to bother him. "He was made out to be selfish in the film, and he was self-absorbed, but he changed, he became almost a family man. He used to do the lawn and keep his eye on my children – he loved them – and my dog and cat.

"When my car skidded on black ice and we were trapped in the car in a ditch, Vivian insisted that I got out first. That was the point when I realised, 'My God, you've changed.'"

Lister, McKerrell and their close friend Colin Stone carried on the spirit of Withnail and I with a battery of travels and adventures. Stone, an eccentric entrepreneur dubbed "Lord of the gnomes" after making his first million in his teens selling garden gnomes, made and lost several fortunes and eventually served a jail sentence for fraud.

"We were like the three musketeers. It was unbelievable. We went everywhere we wanted, did whatever we wanted." She describes impromptu trips to Edinburgh, the best hotels, hampers of food and alcohol, rock'n'roll on McKerrell's portable stereo. On a trip to see friends in York, McKerrell bought her a beautiful ruby engagement ring.

However, Lister was concerned that McKerrell's voice was becoming hoarse. Although he was still only in his mid thirties, a hospital check-up revealed throat cancer which was treated with radiotherapy. Yet, despite the warnings that the illness could return, he didn't stop drinking.

It was this which in the end caused Lister to end the relationship. "Vivian did drink too much. He would cut down for long periods, but then it would happen again. Even his family called him Vivianis Horizontalis, which is funny, and not funny at the same time."

"People said we were perfectly matched, but I began to pull back from the drinking. I could see it was a demon for him. Once, Viv and I got talking to an older woman in a pub who was waiting for her husband. She said: 'I can't stand my husband, all he does is drink, but I'm too old to do anything about it.'

"Later, the husband arrived – he was very handsome, and a lot older than Viv, but somehow I could see myself in that older woman."

When they split, he begged her not to leave. She says she realises now that she was the love of his life. He went back to acting for a time, but was forced to stop when the cancer returned. When Lister saw him again, several years later, she was horrified. Surgery had removed his voicebox, leaving him unable to speak or swallow.

"I think he wanted to die. He could only feed himself through a tube, so he would pump sherry and whisky through it so he could be drunk all the time. After Peter Young and I went to see him, we stood outside the room crying." With their encouragement, McKerrell accepted a place in a hospital, and gradually regained something of his former spirit. She believes they gave him another five years of life.

Bruce Robinson took him to see Withnail and I when it was released in 1987.

"He loved the film, he thought it was very true, but I think it also made him rather sad," Lister says. "A lot of friends thought that Bruce should have used Vivian in the role, but Bruce knew that Viv was in the grip of alcohol too much by then."

Lister also loves the film. "It was weird watching it the first time. It was amazing listening to the dialogue, the sense of (Bruce and Vivian] comes across very clearly. It brought it all back, and whenever I see the film now it gets even stronger." Though she went on to have many adventures, and is now resuming her work as a portrait painter in Scotland, she talks about McKerrell with a hint of regret.

"Bruce has done for Viv what Viv would never have done for himself, he made him into a well-known person. I think Bruce did that deliberately. I think he wanted to capture that unusual friendship they had. One of our friends said Vivian didn't need to star in films because he was a star in his own lifetime. He didn't need to be anything but alive."

She tells McKerrell's story now as a tribute to him, but also as a cautionary tale. "I feel that youngsters who want to be Withnail should have seen Viv when he was so ill. They wouldn't want to go down that road if they'd seen what it can do to you. He was a fascinating person but he shouldn't be a role model.

"We would have him sitting here now if it hadn't been for the drink." sm

- Withnail & Me, an exhibition of photographs by Murray Close taken during the making of the movie, is on show at the Proud Galleries, Camden, London NW1, until 7 June (www.proud.co.uk)