It occurred in the early Seventies, when the pair had been an item for eight years, during preparation for the follow-up to the film The Boy Friend ("You remember, lovely film, really made Twigs"). De Villeneuve blames it all on a contractual problem. A script had been arranged, a producer engaged, Fred Astaire was going to star alongside Twiggy and a semi-famous actor was lined up to play the love interest. But then the snag struck: the money couldn't be agreed on, and he dropped out. So an actor called Michael Whitney was recruited in his place. And, whoops.
"Halfway through the filming she rings and tells me she's in love with Michael Whitney and I've got the old elbow. Devastated I was," De Villeneuve remembers. "But the thing was, it need never have happened. Michael Whitney never needed to be there. It was contractual problems, you see, with the first geezer. Contractual problems which I could've sorted, but the agents wouldn't let me. If only I'd been allowed to sit down with the bloke, then, well: crash, bang, wallop, two kippers and a bon-bon, how's your father, done and dusted." A colourful way of recalling his life has Justin de Villeneuve. So colourful, indeed, that he has put it to music, in a production called A Fake's Progress, which opened last night in the West End. The show stars Paul King, once of the pop group King and now of MTV, who picks his way through the De Villeneuve story wearing a kabuki mask.
"I've put him in a mask," says De Villeneuve, "because what I'm saying in the play is everyone wears masks. You see, though the story is based on my life, it's not me you see on the stage. It's a character I invented, a mask. Though in many ways, I suppose, I invented myself."
It is hard to reconcile the elegant man of nearly 60 that he is now, dressed, in a sort of meteorological insurance policy, in both sunglasses and a large raincoat, with the Justin de Villeneuve of the Sixties. But back then he was the sharpest wheeler-dealer around, a man who makes Eric Hall, the football agent, look calm, a man for whom the term "wide" might have been coined.
It started with the name. Nigel Jonathan Davies isn't much of a moniker for a mover and shaker, but that's what he was landed with when he was born in Hackney just before the war. After being evacuated and billeted with JB Priestley during the Blitz (he used that experience for one of his lyrics: "I've seen a better way/ Dined on fine souffle") he returned to the East End and started blagging, calling himself Nagels. Everyone, he says, with their fingers in dodgy pies, knew Nagels. He was one of the most persuasive mouths around, learning his craft by encouraging punters into strip clubs in Soho (another couplet: "Step inside Nuderama/ If you're looking for sex and drama"). From there he turned a few folding ones as the plant in the audience who volunteers to fight the boxer in the fairground ring and soon he was buying and selling this and that. Once he got his hands on a job lot of wine.
"It was an insurance job, Jewish lightning struck the warehouse," he remembers. "The wine tasted like paint-stripper, but I just stuck fancy labels on it, and sold the lot to Vidal Sassoon for his wedding. He invited me along, I was very nervous, positioned myself by the door to do a runner. But they were such gullibles, his guests, they got conned by the labels, everyone loved it. Vidal was so impressed, he made me his assistant."
It was then, as number two to the fanciest snipper in town, that the young fast-lip decided that a false name - a nom de perm as it were - was required to complete the con.
"I mean Nigel, it's usually a name carried by 18-carat prats," Justin claims. "Nigel Mansell: I rest my case. I'd heard the name Justin and I liked it. Then someone said I should choose a French second name, but I didn't know any. So they said, 'Well just take the name of a town.' So I said, 'What, like Harlow New Town?' And that was it: Villeneuve." And so, suitably titled, the young hairdresser soon found himself blagging his way into the affections of all sorts of handsome women who came in to have their hair done. One was a skinny 15-year-old called Lesley Hornby. Within six months of meeting her, he'd changed her name to Twiggy and she was on the front cover of every magazine, often snapped by Justin himself, now passing himself off as a fashion photographer.
"A lot of tap dancing went on," he says. "I didn't know what I was doing, I had a lot of front, but I had a lot of taste as well. I'd only let her do the good stuff. We exploded, we were like the biggest names. It was Mick and Keith, John and Paul, and Justin and Twigs. At one point I got through 23 cars in 12 months, Ferraris the lot. I had a domestic staff, five of them: cook, butler, chauffeur. Ridiculous, but I don't regret a minute of it."
And then along came Michael Whitney and out went his main source of income. For a few years, De Villeneuve traded on the old name, doing a bit of interior decorating here ("Some poor sap in the City gave me a grand for doing out their restaurant; I hadn't a clue"), a bit of pop management there (two of his clients were Tim Hardin and - we all make mistakes - Clifford T Ward). But it was not enough to sustain the domestics.
"I was used to picking up the phone and making it work," he says. "All of a sudden you've got receptionists saying, 'How do you spell that name?' "
Then, in the middle of a very barren run in the Eighties, came a crushing revelation. "I realised that I was only any good at that sort of thing when I was with Twigs," he says. "It became an enormous handicap, my name. You could feel people assuming everything I did must be a load of old bollocks. People seemed to take enormous pleasure in my fall."
Moreover, by then, Twiggy wasn't around to help him make a few quid. "I haven't laid eyes on the woman in 20 years, our paths never cross," her former mentor says. "But one of my daughters goes to Bedales and I think she moves in the same social circles as Twigs's daughter."
All this self-assessment, though, had a positive outcome. De Villeneuve decided to write about the good times, first in book form (An Affectionate Punch, published in 1986) and then as a musical.
"I still only work with top people," he says. "I've got Bob Carlton who did Forbidden Planet directing it, and Paul King brings a saturnine darkness to it."
The musical, he believes, will finally bring about a change in the way he is perceived. "What I'm hoping is people think of me as a new name: 'Ah, Justin, the songwriter.' And not Mr Twiggy any more."
'A Fake's Progress' is at the Cochrane Theatre, London WC1 (Booking: 0171-242 7040)