Bonello took a more considered approach with his . "They wanted to do the race," he says, sitting in UniFrance's breezy pavilion at Cannes. "And me, I wanted to spend time on my work, not to rush. Of course, it's more difficult to come second - but I didn't want to come first and do a bad film. I prefer to take my time. I'm slow … For me the film has to be really precise and for this you need time. So I assumed we'd be second."

He was wise to wait, with scoring a place in official competition at this year's Cannes Film Festival, and also latterly being named to represent France in the Oscar race for best foreign language film. Still, Gaspard Ulliel, who plays Saint Laurent in Bonello's film, admits it was "weird" knowing that another actor was approaching the same character at the same time. "I was constantly wondering how far he would go, how he would change his voice - and if he would change his voice," the star says. "And then I realised that I had to totally forget about this other biopic and just focus on ours."

While Bonello claims he has not seen , partly due to being so busy finishing up his movie, Ulliel did see it after playing the role. "I was relieved when I saw the film," he admits. "I could see the two projects were totally different; it was two different visions of this character - two totally different directions. The two projects never meet at any point. So in a way, it's fair for both of the films to exist."

The actor praises some aspects of Lespert's biopic - notably Niney's performance - yet can't help but make the odd dig. "I hate the scene where he creates the Mondrian dress when he just pulls out a book from a shelf," he says, referring to the Saint Laurent garment inspired by the work of Dutch painter Piet Mondrian. "I think it's the worst."

In Bonello's version, there is also less interest in Saint Laurent's troubled romance with Pierre Bergé, the shrewd businessman behind the YSL fashion empire.

"My real subject is Yves. It's not Pierre and Yves," says the filmmaker, who begins the story proper in 1967, when the designer and Bergé (played by Jérémie Renier) had already been together for eight years (a tactic that also misses out his early work for Christian Dior).

More in focus is Saint Laurent's love affair with Karl Lagerfeld model Jacques de Bauscher (Louis Garrel), which may explain why the real Bergé decided against offering assistance to the production. Bonello, though, is unable - or unwilling - to explain this strict blockade. "Ask him," he says, shrugging, evidently miffed.

It meant that, unlike Lespert's "authorised" version, the production could not raid the YSL archive that has been carefully cultivated by Bergé. "I don't care to have the real stuff - his real glasses or whatever," Bonello says. "We had to dream a bit more."

Certainly, Anaïs Romand, 's costume designer (who also worked on Bonello's 2011 film, ), can be credited with miraculously recreating YSL's extraordinary garments with no official assistance.

Whatever the reason for Bergé's decision, Bonello is still fully aware of how important he was in Saint Laurent's life. "Without Pierre, Yves would not be Yves. That's obvious. He allowed Yves to concentrate on the creation, taking care of all the rest. They built something together, so of course he's very important."

Yet it's not hard to sense Bonello is relieved to be able to let the other film do the talking on this matter.

Undeniably, is a film that seems bewitched by the sights and sounds of the 1970s, a particularly hedonistic period for the designer, as he became intoxicated on a cocktail of booze and drugs. "It's one of the most interesting parts of Saint Laurent's life," Ulliel says. "It's when he's at the peak of his success and his creation, and just before … he starts falling."

If Bonello's film does anything well, it's lifting the lid on Saint Laurent's fractured character, the neuroses and delusions that weighed him down. The opening scene - a brief flash-forward to 1974, with Saint Laurent at the height of his fame - sees the designer in a hotel room, conducting a telephone interview and discussing his time fighting in the Algerian war and the subsequent electroshock therapy that he had to endure.

"His entire life was a lot of suffering," says Ulliel. "In the end, I think he was kind of lonely, isolated, and seeking love in all the people around him, seeking comfort and some kind of confidence. I learned that he loved to write letters to many people he was sharing things with - even a lot of models from that time had love letters. So it's somebody who had a lot of emotion to share."

Ulliel, 29, is evidently not afraid of taking on such an icon. He has a history of bravery, having played the young Hannibal Lecter - the serial killer made famous by Anthony Hopkins in 1991's - in 2007's . "I sometimes have a very lazy nature," he says. "I need those big challenges and daunting moments in my life - to maybe try to go beyond and transcend something. Maybe it's unconscious, but I'm attracted by those types of roles."

Bonello worked with Ulliel for three months before he decided the actor was right for the part. "After three months, I knew I was as interested in him as Yves, and I knew we could create this character and make a picture that was alive and not a museum." The director didn't want a lookalike or an imitation. "The way we worked with Gaspard, the character is 50 per cent Yves and 50 per cent him."

And what about the fashion? Did Bonello gain a greater appreciation for the artistry that goes into the world of haute couture? "No, not so much," he says, bluntly, although he stops short of calling it superficial. "That's the image that it gives, because it's just clothing. The speed of everything - the collections, they're in magazines, you buy them, you throw them away. But behind that, there is still some art." And Saint Laurent remains one of its great artists.

Saint Laurent,