“People have to be on their knees in front of me,” whines Valentino to his business partner and companion of 50 years, the long-suffering Giancarlo Giametti. It is around this point in this lightweight but thoroughly enjoyable portrait of “the last great couturier” that you realise that Valentino and Giametti really have forgotten that the camera is trained upon their every niggling sideswipe, the point where a reverential documentary about the creator of some meltingly lovely frocks kicks up a notch and turns into great entertainment.

Although the first screening at the Venice Film Festival was hampered by some fairly serious projection glitches, the reception was warm. Valentino, as the title suggests, is Italian royalty.

Matt Tyrnauer, special correspondent for Vanity Fair magazine, produces and directs the film, which perhaps accounts for the extraordinary level of access that the crew was permitted. Tiny, imperious, but with a flicker of self-aware humour, Valentino performs for the anxious, handheld camera. He dresses an unhappy pug (he has five) in diamond earrings. He makes declarations such as “an evening dress that reveals a woman’s ankles while walking is the most disgusting thing I have ever seen” – and means it.

He has the weathered face of a Galápagos tortoise – an effusive Italian journalist compares him to the Sun King, but you can’t help thinking that the Sun Bed King might be more appropriate. It’s a film far superior to the recent Lagerfeld Confidential, which breathlessly revealed precisely nothing about Valentino’s equally flamboyant contemporary.

An endless parade of celebs floats through Valentino’s perfumed world. Michael Caine and Joan Collins banter; Sarah Jessica Parker stares hungrily at a dress like a starving dog looking at a steak; Liz Hurley chats amiably; Gwyneth Paltrow, Anne Hathaway and sundry models float around; Elton John, for once, is not the centre of attention.

We are introduced to the inner circle, including Valentino’s muse, Princess Rosario of Bulgaria, and his fearsome head seamstress, who seems to exist permanently on the edge of a frock-stomping rage.

But Giametti, with his sad spaniel’s eyes and expressive range of resigned shrugs and eyebrow raises, soon emerges as the key character in the film. Glimpses of the day-to-day workings of the relationship between Giametti and Valentino provide some of the most amusing moments. An argument about the staging of a catwalk show – “the sand dunes look like two tits” – rapidly descends into barbed spite. Giametti closes the argument with a blow below the belt: “Your belly is showing,” he hisses.

But just when the film looks to have settled into the groove of fashion pantomime, it surprises with a couple of genuinely moving moments. During a speech, Valentino offers a choked-up vote of thanks to his lifelong companion Giametti; and later, when a fin de siècle melancholy tinges the lavish celebrations of 45 years in the business and Valentino, assailed from all sides with questions about retirement, is forced to acknowledge that he really is a relic of another, far more glamorous, time.

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