Bill Wyman is sitting alone in the corner of his restaurant Sticky Fingers, patiently waiting, as you might expect. His hair is a weird shade of purply-brown these days. He's skinny with no bum to speak of, there's a fag in his mouth, a pot of tea by his side and, of course, he's on time. Wyman was always known for his promptness. As a Rolling Stone he and drummer Charlie Watts formed the most reliable rhythm section in the world; they never missed a beat. He was the archetypal bass player - still, stony-faced, silent. Wyman was probably the least well-known band member till he became a front-page news story in the 1980s - partly for sleeping with more women than Willie Nelson and Julio Iglesias could dream of, and largely for sleeping with a girl who was 13 years old when they met. Nowadays, he's a family man - happily monogamous, almost 70 and with three young daughters.

It's 15 years since he left the Stones. Since then he has written books about himself and his friend, artist Marc Chagall; he has discovered historical coins as an archaeologist, been a restaurateur and a member of the R&B combo the Rhythm Kings. But, of course, to most people he will always be Wyman of the Stones. He's still the band's archivist, his collection worth millions. Wyman has often said that he was born to be a librarian.

While Mick Jagger and Brian Jones came from the upper middle class, he, Watts and Keith Richards were working class. Wyman grew up William Perks in Penge, south London. He hated the name and the place, thought they both reeked of failure. He was one of only three lads in a class of 52 who passed their 11-plus to go to grammar school. At 15, just before O levels, his dad took him out of school. "I think he did it out of spite. There was that resentment about me going to a posh school trying to talk posh and wearing a uniform that he had to pay for. He was very working class and a very cold person."

William Perks spent the war years with his grandmother, whom he adored. After school, he went to work at a diesel engineer's as a clerk; he did two years' national service in Germany, and played with a rock'n'roll band for three years before joining the then bluesy Stones. He was competent but rarely confident. Until he changed his name. "I did it officially through a solicitor by deed poll in March of 64. It cost me £25. I liked the name Wyman because my best mate in the military was called Lee Whyman. He was the best footballer in the camp, he could dribble his way through anything." It's a sweet story, and typically Wyman - typical in that he remembers exactly how much it cost and typical in that he named himself after somebody he idolised. "It completely changed my life. I felt confident, I was proud of the name."

Wyman was married at the time, with a young boy, Stephen. He was a rubbish husband, mired in an unhappy marriage, so he went off with the Stones and embraced a world of fame and excess. There are some questions that can't be asked delicately, no matter how hard one tries. So I don't. When did you become so shagtastic, Bill? "1963 onwards, really," he answers instantly and po-faced, every inch the archivist. "Me and Brian." What do you mean? "Well, the others didn't do it. Hardly at all." Why not? "Mick and Keith were always in their rooms writing songs, and Charlie was faithful to his wife. I wasn't, because right from the beginning my marriage was a failure so I had no guilt about fooling around."

I tell him that I had read he once slept with 265 women in a three-week period? (Actually, it was a two-year period, but I thought I'd tickle his ego.) "Three weeks?" he laughs, as if the suggestion is absurd. Then he becomes serious and knuckles down to business. "Three months. Yeah, cos you used to have three or four a night sometimes. In Australia, the girls used to stay outside all night long. Me and Brian used to look out of the windows, cos we shared a suite, and we would ask the night porter to go out and get the one in the striped thing and the one in the shorts next to her, and they'd come up, and you'd spend a couple of hours with them and say bye and give 'em a kiss, and then about half an hour later you'd say, 'That one in the red dress.'"

Blimey, I say, semi-shocked, you must have had some stamina. He nods. Life could be so dull on the road - a Groundhog Day existence of plane, gig, hotel, plane, gig, hotel. "They helped get over the boring times. And it became habitual." In a way, he says, it was an addiction, but a relatively safe one. "It was better than drugs because you couldn't OD on it. If you'd had enough your body didn't work any more, and it was as simple as that. So I thought it was quite healthy."

Were Mick and Keith aware of your success with women? "Yeah," he says. He beams. "Yeah, of course, yeah. They used to raid my room. I was in there with three girls and they'd get the bloody night porters to open my room saying it was their room and they'd lost the key and they'd walk in on me in bed with three girls. Yeah. It was rather embarrassing. I used to get pissed off about that."

Blimey, I find myself saying again. (I've never found myself blimeying so often in an interview before.) Did they ask you what the secret of your success was? "Nah. The Stones never talked on a deep personal level to each other. Me and Charlie do, but not the others. It's all peripheral stuff, unless it's Stones business." (He hasn't a word to say against Watts, his closest friend who he still sees two or three times a week.)

Well, I'll ask for them - what was the secret of your success? He enjoys the question. It's something he has often thought about. "When I worked in that diesel engineer's, a friend said if you ever want to be successful with women treat them all like ladies, every one of them. Whether you think they are ladies or not, treat them all very respectfully and nicely."

It's an unfortunate explanation in light of Mandy Smith. Smith certainly wasn't a lady when they met - she was a barely teenage girl. Eventually, they married, when she was 19 and he was 53. Not surprisingly, the relationship became front page news. He has said in the past that he was lucky not to go to jail for having sex with a minor.

How did you get yourself into that situation? "I don't know, mate," he mumbles. "I don't know. It's the magic of the moment, basically. It was 1984 when I met her. We got married in 1989. That was a disaster." He is mumbling, but to his credit he is trying to answer honestly. Did he ever think it could work out? "In 1985-86, I did, yeah. Then she went off the rails a bit. She had a lot of affairs and got really ill. Got really thin. Almost died." He doesn't say as much, but I sense he thinks of himself as a victim of fate.

The situation became even more bizarre when his son Stephen began a relationship with Mandy's mum at the same time. Does Wyman ever see Mandy? "Nope. Never. It was an error I made. It was something from the heart more than anything else, and it was a mistake. I think everybody makes a mistake; it's just when you're a public figure it gets around much more than if you're a fuckin' garage hand in Harrow or something."

It wasn't simply his relationship with the band, though. It was his failing relationship with Mandy that led to his leaving the Stones. "I had to get my personal life in order. That was really necessary." He had made a complete mess of everything? "Yep, so I had to get rid of everything and turn over a new leaf and start again." Ultimately, he says, his relationship with Mandy forced him to grow up. And he went about the business of rebuilding his life with astonishing pragmatism. He approached an old girlfriend, Suzanne Accosta, said he was finally settling down and asked her to marry him. Perhaps, even more astonishingly, his third marriage has worked.

There's an openness and naivete to Wyman. I tell him that he's so different from Jagger. Jagger is cold and closed, but Wyman's history pours out like so much gunk. "Yeah, Mick will never answer a question properly. He doesn't like to talk about the past. He can't stay in one place for more than a couple of weeks, and then he has to be off somewhere else. I don't think he's happy."

When they meet up, he says, he's never sure which Jagger he's going to meet. "Keith used to say Mick's a lovely bunch of guys. Heeehehehee! I can go in a room and he will be, 'Hi Bill, lovely to see you, man!', give me a hug and all that. Then I can go in a room three days later when there's a few celebs there and he don't even come over to talk because whoever's in the room. It's bizarre."

It sounds like insecurity to me, I say. "Well, I think all the band are insecure," he says. That's a surprise. How? "We weren't fabulous musicians. If another great drummer came in and hung out, Charlie would be great mates with them, but when he went on stage and they were behind him he'd shit himself. And I used to be like that when Jack Bruce or someone like that was in the audience, and Mick used to be like it when there was a great singer and Keith would be when Ry Cooder was around."

They were not only ferociously competitive with other bands, they were with each other. When Wyman had a hit single with (Si Si) Je Suis un Rock Star, Jagger told him it was a "silly record". He says both Jagger and Richards had huge egos, but he talks of the latter more affectionately. (Richards, for example, told him he liked Si Si ... and played nicely with Wyman's girls when he took them to see the Stones a few years ago.)

At times, when he talks about the Stones, he sounds like somebody who has escaped an abusive relationship. He was not allowed to write songs for the band ("Sorry mate, we write the songs for this band," he was told) and received no publishing royalties for the music. (The Beatles split royalties despite Lennon and McCartney writing most of the songs.) Today, Wyman still resents the fact that he was treated as a hired hand.

When he talks about having to be careful with money and not being as wealthy as he should be, he sounds like a pensioned-off Spinal Tapper. You must be worth around £30m, I say. "Are you serious?" he asks. "Whatcha talking about? With properties and everything? Nah. Maybe £20m. But cash in hand is the problem and I've got very little of that. So I've got to work. I can't stop." He has always voted Tory, "because they are the ones that looked after me. Labour? Come on. We had to leave the country didn't we? It was 93% tax. After paying millions into the Inland Revenue, you know what they gave me for a pension?" He pauses dramatically. "£34 a week." And he sniffs his contempt. The country's going to the dogs, he says.

But he knows that sounds churlish. Actually, he says, life has been fantastic over the past 15 years. He tots up the number of things he has done - the writing, the archaeology, the restaurant, playing with his band the Rhythm Kings, the relationship with his son Stephen (who runs his website), marriage, his three daughters. He has even released a compilation of his solo work and post-Stones work, though he admits it would be pushing it to call it a greatest hits album.

He walks me round Sticky Fingers, showing off some of his favourite memorabilia - there is Brian Jones' gold guitar, here is the mini-bass that Wyman built himself - and he stops at a beautiful collection of pencil portraits of the Stones from the mid-1960s drawn by a teenage fan shortly before she died. All five Stones look so fresh in the pictures, 40 years back. Can he ever imagine them splitting up?

"I can't see them going on too much longer, I really can't. It's getting a bit ridiculous, innit?" Because they are becoming a parody of themselves? "Yes, not that I mind. It's their lives. But I don't know if they've got anything else that's important to 'em outside that apart from families." He's thinking about death. It's strange, he says, he's never been around when those closest to him have died. His grandmother died when he was in the military, dad when he was touring in Japan, mum when he was in France. In a way, he says, he has only been able to mourn them in the post-Stones years. Is he an emotional man? "Yeah. I suppose I'm a bit sensitive, yeah."

In some ways Wyman is just as I'd expected, in others very different. How d'you mean, he asks. I expected you to be a right dour bastard, I say. He smiles. "Well, come on, in the press for 30 years they've called me wrinkly, crotchety, grumbly, now look at my face ... I'm not wrinkly. I've got nice soft skin." And it's true, he has.

· Bill Wyman: A Stone Alone - the Solo Anthology 1974-2002 is released on Sanctuary Records on March 20.