The Rollers were globally huge in a way that few British bands truly are any more, selling hundreds of millions of records, scoring a number one in America, causing hysteria from Toronto to Tokyo and back. So to be sitting having a drink with the man who sang the songs the world loved is quite a thrill, even if he doesn’t look much like his teenage self these days.

I meet McKeown in a beer garden, near his home in a fashionably bohemian part of London’s east end. We’re five minutes away from the White Cube gallery, which is showing the new exhibition by his pal Damien Hirst; there are vague plans for the artist to design an album cover. It’s tempting to see something of his predicament in Hirst’s pickled shark. In the preserving public consciousness, McKeown is forever the tartan-trimmed teen idol, a toothy grin frozen on his face, whereas in real-life he is a grown man of 47, slightly bloated, a bit bashed about from years of swimming with showbiz sharks.

He settles into a chair and orders a large Wild Turkey, “splash of Coke, loads of ice”; when I return with our drinks he leans forward with the nervous anticipation of someone who is visiting confession for the first time in years. He wants to talk, and once he starts he can’t stop. It’s all earthy stuff, some of it too deep and dark and unprovable to print. They say that you can smell lies on people, but with McKeown it’s the truth that stinks – rank and mushroomy, rotten from years in the dark. Not all of his stories are like that. Some of them are like out-takes from a great lost bedroom farce, Carry On Rollering. He’s the sort of guy you can ask “So, Les, how many women did you sleep with between 1973 and 1978?” without getting a pint poured over your head.

The answer, by the way, is that he wasn’t counting, but “there were certainly a lot of quickies”.

There had to be. As McKeown writes in his forthcoming memoir, Shang-A-Lang, groupies were verboten by order of the Rollers’ manager Tam Paton.

“It was really stupid,” says McKeown. “Some of the security men would help as much as they could, but they didn’t want to get fired either. You would do it in hotel service elevators, linen cupboards, all the best romantic places. If you ever did get in a room and Tam Paton would come by, you would be stuffing two of them in a closet and three of them in a bathroom.”

It’s amazing, hearing this stuff, to think that the Bay City Rollers were sold to the world as the squeakiest clean band going. They fronted an anti-smoking campaign, the world was informed that they only ever drank milk, Stuart “Woody” Wood was said to be so afraid of the dark that he slept with a teddy bear.

In fact, according to McKeown, life in the Rollers was little different from life in any band of the 1970s in that sex and drugs were available in significant quantities to those who wanted to indulge. And McKeown wanted to very much. He counts himself lucky that his “binge-shag” days were pre-Aids. The worst he had to deal with was a nasty case of crabs, and that was from dirty sheets, although he turned it to his advantage by putting the not-so-tim’rous beasties into Paton’s bed.

There remains a great deal of bad blood between McKeown and his ex-manager. And he still has genuine hatred for at least one former bandmate. Throughout his time in the Rollers, McKeown was engaged in a power struggle with guitarist Eric Faulkner. The great tragedy of the Rollers is that they were a real band who came to be perceived – and sold – as a teenybop outfit, and it seems that Faulkner in particular found this hard to take. He was always keen that more of his songs should be included on the albums, much to the chagrin of McKeown, who believed that these originals simply weren’t as good as the cover versions and material by professional songwriters which first brought them success.

“Eric Faulkner really did think he was John Lennon born again,” says McKeown. “And you know, he has never written anything that’s worth spitting on. But his attitude, his pomposity is unreal.”

Faulkner may not have been the reincarnation of Lennon, but the Bay City Rollers phenomenon did rival Beatlemania in its intensity. Though they formed in 1968, and had a top 10 hit three years later with Keep On Dancing, it wasn’t until 18-year-old McKeown replaced original lead singer Nobby Clark in 1973 that they enjoyed lasting success. From 1974 through to 1976 they were the biggest band in Britain, causing hysteria at every concert and public appearance. “We got mentioned at the despatch box,” says McKeown. “We got banned from playing in the UK for about three months. We were considered too sexy for Britain.”

The three singles they released in 1975 – Bye Bye Baby, Give A Little Love and Money Honey – went to numbers one, one and three respectively. Of course, these are just empty statistics, and it’s worth digging out the press cuttings from that year to see just how surreal things became.

On May 5, 1975, The Guardian made an anthropological study of Rollers fans at a concert in Leeds: “Some girls have faint scars on their forearms where they have drawn blood while scratching ‘Eric’ or ‘Les’ with a pin – the stigmata of pop. ‘Mine turned septic,’ said one girl, ‘and me mum was right mad.’”

May 8: The Rollers are invited to perform live, on a boat, in an ornamental lake in the middle of the Mallory Park race track as part of a Radio One event. Forty-seven thousand people turn up to see them. Forty girls are rescued by police in rowing boats, and four are hospitalised.

May 27: The Herald reports that “Mr John Ross, rector of Wick High School, yesterday sent home nine boys who defied his ban on the wearing of tartan gear identified with the Bay City Rollers.”

September 12, 1975: Mr Alec Turner, senior depute director of environmental health for Edinburgh says that listening to the Rollers could bring on premature deafness.

October 28, 1975: The Communist magazine Sovietskaya Kultura announces that the Rollers, together with “sadistic films and pornographic literature” are part of a big-business plot to stupefy the masses.

December 20, 1975: Newspapers report that Margaret Ness, a 15-year-old Rollers fan required stitches after being shot in the forehead with an airgun while sitting on the wall outside McKeown’s home in Torphichen, West Lothian. McKeown is charged with the shooting but later cleared.

But the most astonishing story of all, right in the middle of all this hype and mania, came when McKeown killed a woman while driving in Edinburgh. Euphemia Clunie, 76, died on May 29, 1976, when she was hit by McKeown’s blue Ford Mustang while crossing Corstorphine Road.

Hordes of hysterical girls attended McKeown’s trial, screaming with delight when he was cleared of causing Clunie’s death. He was found guilty of the lesser charge of driving recklessly and dangerously, fined £150 and banned for a year.

“I can’t really tell you what I felt about killing that old woman on the day or the day after,” he says now. “I just know that it’s always with me, and I wish it never happened. But it was a genuine accident.”

On the night of the crash, the Bay City Rollers were scheduled to perform in Bristol. The gig was cancelled but the rest of the tour went ahead. “I was 19 years old, I’d just killed someone, and it seemed like everyone around me was pretending it hadn’t happened or it didn’t matter,” McKeown writes in Shang-A-Lang.

He got through it, but broke down at a gig in Oxford. “I was singing some song and I just lost the plot,” he tells me. “I burst into tears, couldn’t handle it anymore. There were all these fans and when I started crying they came forward wanting to mother me. And then my attention suddenly focused on the orchestra pit. There were girls coming over and getting hurt and all these photographers were taking pictures of them, and for some reason in my twisted little mind I thought that was out of order so I jumped into the orchestra pit and started beating up a photographer. Promptly got arrested and got a two-year suspended jail sentence for that as well.”

I ask McKeown whether he wishes he had been sent to prison for causing Clunie’s death. After all, an enforced break might have been just what he needed. He says not, but admits that the car accident and its aftermath marked the beginning of his decline. “It was after that that I slowly started getting into all different kinds of drugs. I dabbled in smack. I really went off the edge into my own private hell.”

In his introduction to Shang-A-Lang, Irvine Welsh makes the point that McKeown’s behaviour was perfectly natural for a young man from an Edinburgh council scheme; suddenly chancing on fame and money after growing up in expectation of anonymity and poverty would be enough to turn anyone’s head.

“I think I was basically just a cheeky little git,” McKeown says when I ask about this, “I didn’t like to take any abuse, basically. If someone was cheeky to you, you were cheeky back to them; if someone needed a wallop, you would get in there, y’know? I suppose growing up in an Irish ghetto like Broomhouse, that’s what you’ve got to do. I’m no tough guy but I don’t take shit from people.”

By 1978, the Bay City Rollers had decamped to Los Angeles. Britain was bored with them by then, but America still liked them enough to give them a TV show. McKeown spent his off-camera hours taking cocaine with Led Zeppelin’s John Bonham and The Who’s Keith Moon, both of whom would be dead by 1980. Just a few years earlier, McKeown had been banging his head in the front row when Led Zep played the King’s Theatre in Edinburgh, and here he was actually taking drugs with his heroes. And then there were the women. McKeown went to a few film premieres with Jodie Foster, but “I never really got into her drawers or anything.”

His later relationship with Britt Ekland was less platonic. “In Shang-A-Lang you write that it ended because you both wanted different things,” I say to him, “but wasn’t the problem that what you wanted was to go to bed with her daughter, Victoria?”

“I did go to bed with her daughter,” he laughs. “She was hot to trot, she was. I don’t know if she did that with all her mum’s boyfriends, but she was certainly wanting a good shag.”

Meanwhile, relations between McKeown and the rest of the Rollers reached an all-time low. He sent them a letter saying that he didn’t agree with their plans for the future and that they should all leave the band immediately; “They wrote back saying ‘F**k you, you’re fired.’”

At 22, it was frightening to contemplate what he was going to do with the rest of his life, but “I was in what I considered a sinking ship full of rotten ratbags. And even if I had done nothing, I wanted to get out”.

He went solo. Legal wrangles meant his career was stillborn in Britain, although he enjoyed a lot of success in Japan. This was fortunate, as McKeown had discovered he was $24,000 (£18,000) in debt from his stay in LA, and, thanks to some incomprehensible accounting, the royalties earned by the Rollers seemed to have vanished. After five years of stardom, he was skint.

We talk a little bit about Robbie Williams, “a kindred spirit”. Williams quit Take That in 1996 before embarking on a solo career which, after a faltering start, has eclipsed his former success. Like McKeown, he is a complex figure – part egomaniac, part extremely vulnerable. The difference is that public relations experts and the popular media find it easier to deal with such complexity these days, and McKeown believes that if he were 23 again now, he might have more success than he had. Headstrong talent quits boy band for life of drug use, womanising and introspection was a recipe for success in 1996; in 1978 it was a disaster. When McKeown talks about Robbie Williams, it reminds me of when George Best spoke to me about David Beckham – there’s lots of envy and regret.

In 1983 he married his Japanese girlfriend Keiko, known as Peko, and in 1984 they had a son, Jube, now known as Richard. By 1986, following a Rollers reunion tour, McKeown admitted he had a cocaine problem and kicked the drug.

He seems to have managed to stay off it too, until quite recently. His parents both died last year, first his father then his mother. Following his father’s death, he started taking cocaine again. He has been taking it this year, on and off, but is currently off it, and has been prescribed anti-depressants to help him cope with his grief.

“Right now, I’m clean but there’s always the possibility of a relapse,” he says. “You just sort of think ‘Ah f**k it, I’ll buy an ounce and tan it’. While you’re doing it, you’re right exactly where you want to be, right in the thick of all the misery and wallowing in self-pity. And then I don’t know what happens, you snap out of it and you get a smile on your face, and suddenly you’re high as a kite without any drugs.”

We talk about his parents, Florence and Francis, to whom Shang-A-Lang is dedicated. He has been talking to a psychiatrist, “but there’s nothing he can say or do to take away the endless sorrow”.

After they died, McKeown spent a bit of time living in their home in Edinburgh. “I do get very drunk when I’m up there,” he says. “I just sit and drink and talk to ghosts. I sit and think what it would be like to be my mum or my dad sitting in that wee flat. It kind of makes me sad to think that I could have made their life a wee bit more interesting by spending some more time with them. Although I did spend time with them, but even more.”

When he tells me this, McKeown looks like he is close to tears, and I have to admit feeling sorry for him. He is not what you would call a nice man – boastful, sexist, full of bile – but it does seem that after the royal flush of the mid-1970s, fate has dealt him a succession of bad hands. Fame Academy should have him on their panel of judges; he could be a kind of Jacob Marley, a wraithlike figure from the past, materialising to warn today’s pop hopefuls that the idea of being famous may seem heavenly, but not being famous anymore is hell and purgatory bundled together.

Still, McKeown has not fallen out of love with music. He plays guitar in a band called Damaged, happily letting the singer take the spotlight. He is soon off to Australia on a tour which he hopes will give him an opportunity to really clean up, even to quit smoking. He is thinking of setting up his own children’s charity, an umbrella organisation dealing with underprivileged and abused kids.

Since his parents died, McKeown has been thinking of trying to reconcile with his brothers Brian and Roni, neither of whom he has much to do with at the moment. His other brother Hari looms large in his thoughts. Around five years ago, Hari confided that he had been raped at the age of 12 on his first night in Gilmerton remand home. Moving to the Mussbank Approved School in Glasgow, he was “gang-f***ed” by fellow internees. McKeown says Hari’s life was skewed by these events, culminating in a prison stay in the late 1990s, during which time he was attacked with a hammer and, according to McKeown, suffered a broken jaw, fractured skull, permanent damage to his spinal chord and loss of feeling in his face, three broken ribs, a broken collar bone and lost four pints of blood.

One positive in McKeown’s life is that over the last few years he has grown closer to Hari, who he says is really a very gentle and talented person, held back by the murk of his past. “He deserves a bit of sunshine in his life, but I don’t know, it might blind him.”

You could say the same about McKeown. “I owe it to myself not to screw my life up,” he says, finally, and as I wish him bye-bye I’m hoping that he hasn’t left it too late.

Shang-A-Lang: Life As An International Pop Idol is published by Mainstream on October 20