"I had committed symbolic parricide while still dithering about the real one," claims Horsley, who insists that you don't have to be Freud - "and I'm talking Clement Freud here," he chortles - to work out that his passionate sexual and emotional involvement with Glasgow's most notorious hardman was all to do with his desperate search for a father figure, since his own late father was "a drunk and a cripple".
Even more shocking, though, than any homosexual liaison with Boyle - who famously turned from criminal to artist while incarcerated in the Special Unit at Barlinnie - is the notion that Horsley's late wife Evlynn was also involved in an adulterous affair with Boyle at the same time. "This was a menagerie trois," says the man who made a million in the art market and then spent it in a year.
The controversial Soho-based Horsley, an artist manqu, a ridiculously rude and funny writer and a self-described peacock without a cause - who has been called everything from "a posturing popinjay" and "a neon narcissist" to "a pervert who stands for everything that is wrong with British society today" - reveals this and much more in his scabrous, scatological and exceedingly strange memoir, Dandy in the Underworld.
In this "unauthorised autobiography," Horsley, who insists that he is not gay despite his claimed relationship with Boyle, writes that he is for ever "poised between Savile Row and Death Row".
This 45-year-old former multiple class-A drug addict, whose pink suede designer suits used to have secret pockets in which he could store his heroin syringes without ruining the cut of his finery, and who claims to have slept with more than 1,000 prostitutes and sold his body to the highest bidder, is the man who had himself crucified "with real nails" in a ceremony in the Philippines in order to make a piece artwork. He also went swimming with great white sharks because he wanted to paint them.
"I've suffered for my art - now it's your turn," he warns readers of his book, which is often repulsively funny and yet profoundly sad. He says he decided to write the book because he was "so sick of all those f***ing misery memoirs, where everybody is ill or dying. We live in this culture where everybody has to have an alibi because they weren't loved by either their parents or their bisexual polar bears. The truth is, darling, I wasn't sexually abused as a boy - I was a plain boy and nobody so much as looked at me. So I didn't want to write a book that was 'Poor Me'."
For all his protestations, his loveless childhood actually sounds horrendous, despite the immense privilege into which he was born and the facetious way he writes about having a mother who was often to be found lying in a pool of her own vomit, and a father who was completely inaccessible.
However, it's the scandalous revelations about his enslavement to Boyle that could make Dandy in the Underworld one of this year's most talked-about books.
At 18, Hull-born Horsley, whose family were the proprietors of the vast Northern Foods empire, moved to Edinburgh to read English at University. First, though, he had to resit his French O-level at Stevenson's College, in Wester Hailes, where he met the anarchic Boyle in 1980, rapidly falling under his charismatic spell. Horsley's grandfather introduced them, after putting on several exhibitions of Boyle's sculptures in Hull.
Soon Horsley was in thrall to Boyle, who was on a Training For Freedom course, working two days a week at the local community centre, then returning to Saughton prison at night. Horsley had already devoured Boyle's book Sense of Freedom, telling how he had stood trial for murder twice in 1965 - on the first occasion he was found not guilty and on the second the charges were dropped. The houses of witnesses involved in both trials were blown up by gelignite bombs.
In 1967, Boyle was finally jailed for murder after being found guilty of stabbing William "Babs" Rooney to death and given a life sentence. In prison, the convicted killer earned the soubriquet of "Scotland's most violent man". Nonetheless, he began sculpting, and wrote the bestselling Sense of Freedom, which he followed with The Pain of Confinement on his release.
"Sitting opposite him at the community centre in Wester Hailes, I was in awe. And he knew it. He is the only person I have ever known who can still strut while sitting," says Horsley, whose book tells how his own mother - a drunk and a manic depressive, who consumed her own dead father's ashes sprinkled on her porridge - tried to abort him but failed. ("Had she known I would turn out like this she would have taken cyanide," he says.)
"I sat there, like a rabbit, in the headlights of Jimmy's steely blue stare," says six-footer Horsley, who became utterly besotted with Boyle. "Crime, like music, fulfils every boy's fantasy of daring and danger. But for me it was more than this. I didn't just want fantasy, I wanted a father."
Horsley's shock of black hair is arranged into a punky firework explosion, while his fingernails are painted red. He is attired in turquoise velvet. ("It's new, this suit, although I am in arrears with my Savile Row tailor," he admits. "I've just taken delivery of another made entirely of red sequins and one in black sequins, on which my principal hopes of happiness depend, because it could attract attention at a Liberace concert.")
Shortly after meeting Boyle, Horsley fell in love with Edinburgh-born Evlynn, the youngest of five daughters of a decorator and his future wife, who had been brought up in a basement flat in the capital that would have fitted into the pantry of High Hall, the Horsley family seat on Humberside. "She had a face which indulgence was already leading to decay," he writes.
The pair moved to London and Horsley won a place at Central St Martins art school. Later, Horsley and Evlynn, who died several years ago at the age of 40 of a brain tumour, married unhappily, at Boyle's instigation.
Expelled from St Martins, Horsley moved back to Edinburgh - "the grimmest city on earth" - after Boyle was released from prison.
He went to work with Boyle and his wife Sarah - the daughter of the then film censor, John Trevelyan - at the centre they set up in the city, the Gateway Exchange. "It was to be a sort of last-chance saloon. 'People coming out of prison, coming off drugs and those with mental health issues' was the official line," says Horsley, adding that "the hoodlum world of art was its core".
He says that after his intense affair with Boyle ended, he made a failed suicide attempt and had an acute nervous breakdown.
"It was the unhappiest time of my life," he says. He resigned from the Gateway and he and Evlynn divorced. He wrapped his wedding ring in loo paper and flushed it down the lavatory. "It's probably still there, stuck in the U-bend - a symbol of all we had shared."
Later, Evlynn admitted she had been sleeping with Boyle before she and Horsley wed. Meanwhile, he and Boyle had a sort of reconciliation, but it was a relationship so full of secrets and lies that it left him completely stranded.
"There was a lot of love, a lot of passion between Jimmy and I," he says. "I hope that Jimmy will accept that this is the true story of the love affair we had. He's a brilliant man and I was treated badly by him with my own consent," he says. "But I couldn't write my life story without writing about Jimmy. If that means I get murdered, so be it - only joking, darling!"
When his relationship with Boyle ended, he says, he was actually afraid for himself for a while. "I had good reason to be. Scratch a lover and find an enemy. In the past Jimmy had liked hurting people. A lot."
Meanwhile, Horsley developed a fondness for crack and heroin, and prostitutes on whom he reckons he's spent 100,000, while "failing successfully" to become an artist.
Today, he's off drugs, although he still pays for sex. He's been clean for more than two years. "I still have a habit. I would be bereft without one. But I've replaced the habit of using drugs with the habit of not using them - but quietly.
"My existence, however, is a work of art. It deserves a frame - if only to distinguish it from the wallpaper," he says. Almost bankrupt now, he's squandered money like love and is making economies. "I walk to the Ritz; I find extravagance sometimes helps me."
In a stream of Quentin Crisp-style aphorisms he drawls lazily about killing himself one day. "I'll go on for a bit longer, though. My song is not quite sung. London is not yet tired of my wardrobe, for a start.
"I've renounced everything, apart from Satan - and satin. In the long term I'm sure I'll achieve nothing," concludes this immensely likeable man, of whom his friend the novelist Will Self says: "Sebastian is simultaneously enthralling, charming and fantastically annoying." While Horsley remarks of Self: "He has a face that resembles a bag of genitals."
To become an exquisite work of art, confides Horsley, has been the object of his life. As for that shocking crucifixion, which offended so many people and led to the red-tops calling him an "art freak," he says that when he fell off the cross after the footrest collapsed - horrifying onlookers who feared he had died - the resulting pain was something of a relief as it meant he was alive. He adds that he considered calling the subsequent photographic exhibition "Is There a God or Am I Too Fat?"
"Yes," he acknowledges, he is preposterous, vulgar, absurd. "I'm a futile blast of colour in a futile colourless world, a very artificial person. I regret everything. But so what? At least I have cause. But now I'm a reconciled Sebastian. I can allow the arrows to rest gently in my wounds, although there is no redemption in my life."
Some people have told him that Dandy in the Underworld is a "disgusting" book. "I tell them, 'I want to be criticised not admired'. I'm just camping it up, because it's all so pointless anyway, so I might as well be extraordinary. I hope that I've turned my anger into wit and my pain into humour.
"Darling, I'm done with depravity; I'm just a dandy whose next move will be to go to Carlisle and open a knitting shop."
• Dandy in the Underworld by Sebastian Horsley (Sceptre, 12.99). A retrospective of his work, Hookers, Dealers, Tailors, is at Spectrum, Great Titchfield Street, London, from September 6.
JIMMY Boyle, convicted murderer turned sculptor, novelist, and playwright, was born 1944 in the Gorbals area of Glasgow.
Once reputed to be the most violent man in Scotland, in 1967 he was sentenced to life imprisonment for the murder of another gangland figure, William "Babs" Rooney, although Boyle, left with Horsely, denies that he committed this killing. While in the special unit of Barlinnie Prison, he turned to art and wrote an autobiography, A Sense of Freedom.
On his release he moved to Edinburgh where, in 1976, he designed Gulliver, the largest concrete sculpture in Europe. The following year he co-wrote the play The Hardman with Tom McGrath.
Boyle has also published Pain of Confinement: Prison Diaries (1984), and a novel, Hero of the Underworld (1999).
In 1980, he married psychotherapist Sarah Trevelyan, the daughter of former British film censor John Trevelyan. They met in the Special Unit of Glasgow's Barlinnie jail, had two children and separated in 2001. He now lives in France.