HE will go on to speak about the ultimate price of fame - the millions they never got, battles with alcohol addiction and closeted gay affairs - but for now former Bay City Rollers frontman Les McKeown is in a happy place, talking about Christmas.

In a budget hotel on the outskirts of Ayr, McKeown, 59, is telling me about "the Bay City Rollers song that never existed" - a long-forgotten track left gathering dust in the vaults for the best part of 40 years but which he hopes could reignite the spark of his ailing music career.

"I wrote it for Christmas 1975 following the success of Bye, Bye, Baby and Give A Little Love," he says. "I was convinced it could have been a No1 but for whatever reason management didn't want to release it."

The single Rock 'n' Roll It's Christmas Time! - a reworking of an old Rollers demo - was released this month and tomorrow will be D-Day when McKeown will discover whether his long-held dream of a festive pop hit will be realised.

He has teamed up with award-winning Scottish songwriters and producers John McLaughlin and Jud Mahoney of Glasgow-based Boombox, who between them have worked with Busted, Westlife and Michael Jackson.

The collaboration came about after McKeown read a newspaper article in which McLaughlin spoke about his enduring love for the Bay City Rollers and how he would like to see the band make a comeback. The singer admits he was cautious at first. Given his past experiences, it's hardly surprising.

Despite selling more than 70 million records at the height of their success, the Bay City Rollers claim they saw little of the money their music earned: a cautionary tale about the perils of celebrity.

When we meet, however, McKeown possesses the infectious, boundless enthusiasm of a young boy on Christmas Eve. The years fall away from his face as he shares his hopes for the project, the deep emotional investment palpable.

"It would be great if people loved the song and wanted to download it," he says. "If you look at someone like Tom Jones, there was a period where he was considered slightly naff. Then he did songs like Kiss and suddenly he was the bee's knees again.

"There is always a chance that something similar could happen. That's my voice on all those Bay City Rollers songs: millions of people bought them; they got played on the radio all over the world. I'm mega proud of that.

"During my dark years, I was so resentful of it all. If one of our songs came on the radio, I'd say: 'Why don't they stop playing this s****, for f***'s sake?' Now it feels brilliant. I love it."

It is a remark that succinctly sums up how far McKeown has come, a journey which began as a wide-eyed Edinburgh teenager who set out to conquer the pop world and brings us full circle to the middle-aged man sitting across from me, valiantly clinging to the final remnants of a broken dream as tightly as one might attempt to cup sand to prevent it slipping between their fingers.

The best part of four decades has passed since the phenomenon dubbed Rollermania, a dizzying period which saw McKeown and his bandmates - Alan and Derek Longmuir, Stuart Wood and Eric Faulkner - traverse the globe in their trademark tartan ensembles.

The Rollers burst onto the music scene in 1974 with Remember (Sha La La La) and pop anthem Shang-A-Lang. A year later they topped the UK charts with Bye, Bye, Baby and Give A Little Love.

The band appeared in Jackie magazine and on children's TV programmes Crackerjack and the Basil Brush Show. Lovestruck girls camped outside the tenement flat in Broomhouse, Edinburgh, where McKeown grew up.

Not that he was there. Such was their whirlwind schedule, McKeown's mother would later remind him they saw each other only six times in three years as the Rollers juggernaut thundered its way across North America, Japan and Australia.

They racked up hit records with eye-watering ease and saw their dishy, heart-throb images adorn the bedroom walls of millions of girls. In the mid-1970s, their iconic status was comparable to One Direction today. The Rollers should have been made for life. But that's where the story began to unravel.

For McKeown, the rot set in as the band hit the pinnacle of their success in 1976, breaking America where they had a No1 hit with Saturday Night. It should have been a time of joy, yet was marred by dark undercurrents.

"We became Americanised and introduced to all sorts of people who had different ideas about what we could be doing," he says. "It was like: 'Hey, have you ever tried this stuff? It's called cocaine ...'

"I had done a little bit of smoke, but our manager Tam Paton - who is well known for being a complete bastard - had all kinds of drugs. In those days it was Quaaludes [a sedative/hypnotic depressant similar to barbiturates], which were basically sleeping tablets.

"He had other tablets of different colours, greys and reds, which were amphetamines. If someone was tired, they got a couple of pills to help them sleep; if someone needed energy he would give them pills for that.

"They really did push us hard, I would say to the point where they took the piss," continues McKeown. "When they made schedules for us, two things were always missing: time to eat and time to go to the toilet. There was no human touch. We were in a constant mouse-on-a-wheel scenario."

McKeown speaks bitterly of their late manager Paton. "He used to have a house called Little Kellerstain near the former Gogarburn Hospital outside Edinburgh," he says. "In the band we used to call Tam Paton 'the beast of Kellerstain'. We knew even then there was something weird about him and that he was some sort of beast.

"I personally had problems with Tam Paton directly in a sexual way. I stopped him from raping [replacement guitarist] Pat McGlynn but I wasn't ready to come out with what happened to me at that time. I refused to remember it. It wasn't until I went to rehab that all that s*** came up. I tell you what, though, it did help me to get deeply buried secrets like that out and let them go." McKeown claims he was drugged and raped by an associate of Paton on one of the band's early trips to America.

"Tam Paton had lots of friends who were gay. At that time, while the music business was openly gay, that side had to be kept secret from the public."

He believes his first gay experience after being drugged in a hotel room, aged just 19, was pivotal in shaping his sexuality. He describes it as a confusing encounter, leaving him deeply ashamed yet also one from which he derived pleasure. "What happened caused me to bury all that stuff in a deep, shameful way but at the same time, it was an experience that was erotic as well," he says.

McKeown has no such ambivalent feelings about Paton. "I always found it very strange that Tam Paton should be singularly this different type of animal. He wasn't a gay person at all: he was a disgusting beast.

"I don't think he had any actual real relationships, he got his kicks from raping and pillaging. He was pillaging all our money and raping any young lad he could get on drugs basically."

Five years ago, the singer decided to come clean to his wife Peko about a string of gay affairs spanning three decades. During that time, McKeown would meet men for sex, often strangers, with arrangements made over the internet.

Why did he feel he had to bury his sexuality for so long? "It's one of those things I can't really understand myself," he concedes. "It's like all guys, perhaps they don't want to face the fact they might get a hard-on looking at some other geezer.

"When it happened to me, there was a lot of shame. That came not so much from the event itself, but my reaction. I found it erotic and sexually stimulating. That in itself was what I felt shame about."

If his first sexual encounter with a man hadn't occurred in the circumstances it did, might he have felt differently? "I think it could have saved me a lot of heartache and psychological trauma, that's for sure," he says.

McKeown, who has been married to Peko for 30 years, isn't comfortable with the label bisexual. "It would be good if there was another word for it, but I suppose if there is a pigeon hole that would be it," he says. "Let's say I'm not an active whatever it is. I'm back with my wife and everything is cool. I know that I'm capable of loving both sexes. I've not been in love with a man but I have been turned on by the whole eroticism of male sex."

The couple reconciled after McKeown returned from rehab in February 2009 at a facility called Passages in Malibu, California. "I did four months and it wasn't instantaneous," he says. "In the beginning I didn't want to be rehabilitated. I thought they were all a bunch of weirdo, f****** hippies. When I learned to trust them I started to think: 'Maybe there is something in this'.

"It's not like completing the 12-step programme and admitting: 'I'm an addict'. It's the opposite. They believe you do have control over things and I agree. I think you can make your life as close to perfect as you want it because it's under your control. If you choose to f*** up and start boozing again, you made that choice."

He says Peko and his son Richard, 30, were both surprisingly understanding about his stark confession. "Because they love me," says McKeown. "And I love them - I just got lost in the middle of it. I sort of disappeared. The Leslie I am, and was, did vanish for a while."

He is now able to pinpoint key events that led to his unravelling. "The worst time was 2002 to 2008," says McKeown. "One catalyst was my parents dying, one after the other, in 2002. I did feel lonely, sad and very down. I started drinking alcohol more and more. I would have a headache in the morning which led to me having a hair of the dog. It became a cycle."

Rehab taught him to come to terms with their passing and focus on the happy times. "I was told: 'Don't remember your mother's dead face, remember her when she was alive and singing at home when you were young,'" he says. "It's the same with the Bay City Rollers. They told me I should go on tour and celebrate my successful time with the band."

It is a philosophy he has been able to apply to other areas of his life. "I suppose the next awful, terrible thing that could happen to me would be losing my wife or son. That would be the only two significant things that would be a challenge," he muses. "But I'm sure their memory and all the things they went through together with me, it would be a great injustice to end up on the bottle again. That would be a horrible slap in the face of their memory. Of course, I would be grieving but there would be other ways to cope."

Then there is the matter of the millions the band never got. Asked how much he thinks should have been theirs but was lost, McKeown is blunt. "F****** all of it," he says, his laugh tinged with sadness. "The forensic accountants worked out over $110 million was made between 1974 and 1977. Apart from whatever we spent on the road, we got nothing. Tam Paton certainly thieved a lot of it. When we got control in 1977, I realised it was f***** up so bad, you could employ a team of 100 experts and it would take years to unravel it."

When we catch up a few weeks later, McKeown is in buoyant mood. The Christmas single has been released and he's played a handful of gigs.

"It feels absolutely brilliant," he says. "I'm enjoying playing it live and it's really going down a storm. It's a bit of an old dinosaur, the Bay City Rollers. The name is associated with a band that was so massive that in the end it ate itself up. I would like to show a positive end to all that - I see it as a new beginning."

He has some serious rivals for the Christmas No1, not least the debut single by Ben Haenow, the latest winner from The X Factor stable, as well as offerings from The Peace Collective, Ed Sheeran, Mark Ronson and Sir Bob Geldof's Band Aid 30 Do They Know It's Christmas?

"It's not a level playing field, is it?" he says. "If we all got equal air play, I would say I had a really good chance. But a lot of these songs buy their way into the No1 spot with hundreds of thousands of pounds spent on massive advertising campaigns. I've got a reasonable, but still small budget for some posters in Glasgow.

"The other thing is my audience isn't typically the 'download generation'. They tend to prefer going out to buy the physical record. My fanbase is 40-55 and they might not be as computer literate as the fans of some young band that have grown up on their smartphones."

While recent years have seen McKeown, who lives in London, give all things Christmas a wide berth, releasing the single has stirred some yuletide cheer. "Normally every year I take my wife and son to Chinatown and we have a whole day there eating dim sum, walking around and going to the movies," he says. "We usually pretend Christmas never happened.

"Having the song out has made me feel a bit more festive and brought back fond memories from my childhood when I would watch Top of the Pops and tell my mum, 'That will be me one day.' It also brings back feelings of the fantastic time in 1974/75 when the Bay City Rollers were on the top of the world. This song, whenever I play it, reminds me of that year I wrote it."

Nor does McKeown plan to stop there. "We've got quite a few songs finished already and the next single will come out in February, with an album out before the summer," he says. "There is a blinding summer track on it which might even out-do Bye, Bye, Baby. It all depends whether people take me back into their hearts."

Rock 'n' Roll It's Christmas Time! is available for download through Amazon and Apple.