The Edge of Love (15)

Ah Bohemia! The cigarette smoke in demi-monde drama The Edge of Love comes deep and thick and blue, clustering in corners of Soho pubs or curling luxuriantly in the beams of film projectors. John Maybury's period piece is certainly atmospheric, but it's not quite convincing – either as a picture of 1940s Britain or as an insight into the private life of Dylan Thomas. The film depicts the ill-fated relationship between the poet, his wayward wife Caitlin and his childhood friend Vera Killick, née Phillips, with whom we're told he had a long-standing emotional and sexual entanglement.

Apart from its promise of intimate literary gossip, the film's main selling point is its face-off between UK cinema's reigning It Girls. Keira Knightley – whose mother Sharman Macdonald wrote the screenplay – represents a breezy Welsh variety of 1940s poise as Vera, while Sienna Miller stands for roaring snug-bar raunchiness as Caitlin.

The film depicts the Thomas household as a proto-Beat commune putting the wind up stuffy wartime Britain. Biographers have objected that there is no evidence of any affair between Thomas and Killick, but the film is determined to represent the trio's co-existence as a free-wheeling carnival of the passions. The two women share a love-hate sisterhood, with the faintest nuance of chic sapphism represented by nothing more risqué than a shared bath and the odd giggly snuggle under the eiderdown.

It's clear from the start that the film isn't that interested in realism, as Vera performs a nightclub number on a Tube platform air-raid shelter, with production values that Carmen Miranda would have envied. A recurring motif is a huge stylised close-up of Knightley singing, her lipsticked features opaque as a carnival mask. The whole film, in fact, seems to wear a mask of stylish unreality: the Blitz in 1940 could be the Blitz Club in 1980, with a tarts-and-sailors party in permanent swing.

In mufti, Knightley musters a brittle, girlish vivacity that at least feels vaguely period-specific. Miller, however, never convinces you that her Caitlin has set foot in the 1940s. Whether she's sauntering saucily through clouds of Tommies, or flashing an insouciant stocking-top, Miller's self-congratulatory acting is subordinate to an ongoing Austerity Nostalgia fashion shoot.

As Vera's soldier husband, the bitterly excluded fourth corner of the triangle, Cillian Murphy is effective in his quiet torment. But the real energy comes from Matthew Rhys: his Dylan is full of fruity swagger, even if he ultimately comes across as a cynical, amorous buffoon.

The Edge of Love is all the more disappointing because it's directed by John Maybury, a scion of the 1980s British avant-garde who proved his feel for London low life in his terrific drama about Francis Bacon, Love Is the Devil. Maybury's stylisation makes the film more interesting than it would have been if directed by your average British journeyman, but it finally adds up to earnest heritage romance.