Anyone who doubts Sienna Miller's acting ability will be pleasantly surprised by her performance as Edie Sedgwick, the anorexic Radcliffe girl who became a style legend and a star of Andy Warhol films such as The Chelsea Girls.

The pity is that she appears in so scrappy and unconvincing a film; it gives her less chance than she deserves to complete a fully rounded portrait.

Guy Pearce plays a cold, emotionally dry and blank-eyed Warhol who, having used Edie, seems to throw her away.

And there's Hayden Christensen as Billy Quinn, the singer-songwriter, clearly based on Bob Dylan, who tries to save her from herself. They are presented as lovers, which Dylan denies, although he admits that his album Blonde On Blonde and its song Just Like A Woman had Sedgwick in mind.

The film - which opens in 1965 and ends in 1971, when Sedgwick overdosed and died from barbiturate poisoning at the age of 28 - does little service to Edie or Warhol.

He is presented as an artist sucking energy from those around him in the Factory, and paying them a pittance for the privilege. She is characterised as a well-bred and feisty party girl whose rejection and self-destructive impulses lead to her drug-fuelled end. The hideous abuse she suffered from her parents is hardly mentioned.

Both these assessments are feasible, but director George Hickenlooper doesn't allow us more than a glimpse at the reasons behind such essentially dime-store psychology. Factory Girl is too bitty and too obviously on the surface to engage us fully.

What's more, the New York scene of the Sixties is dispatched with scarcely a hint at the creative renewal that Warhol and his cohorts represented, both as artists and film-makers.

Yet, despite everything, Miller manages to hold the attention throughout. She has, apart from anything else, a wonderful smile, and a capacity to make you believe when it is wiped off her face.

She lets you see why Warhol made her a star, if not why he suddenly deserted her. Was it her own fault or not? We are never really told. Perhaps there isn't a clear answer - but it is surely the job of a film-maker to attempt one.