To say that Filth is the best Irvine Welsh adaptation since Trainspotting is to damn with the faintest imaginable praise. Nobody is likely to confuse The Acid House or Irvine Welsh's Ecstasy with ornaments of British cinema.

The new film's success is largely down to a towering performance by an uncharacteristically Machiavellian James McAvoy. Playing an Edinburgh copper deeply embroiled in corruption, racism, sexual perversion and old-fashioned drunkenness, the Scottish actor helps transform a somewhat ramshackle property into an effective contemporary Rake's Progress. Filth is rarely subtle. But it's never boring.

The film hangs around detective sergeant Bruce Robertson’s attempts to manoeuvre his way towards a looming promotion. Rumours are spread about one candidate’s sexuality. A trip to Hamburg lures another into decadence. Bruce carries on a sado-masochistic affair with a third colleague’s wife. And so the grubbiness continues.

This unattractively mounted film swings uncomfortably from rough verité to a school of surreal excess that might have given Ken Russell pause for thought. Jim Broadbent is exhausting as Bruce's inner voice. David Soul turns up in a cameo that accidentally says more about the cruelty of passing time than does any Philip Larkin poem. But McAvoy's leering, slavering, ultimately poignant turn provides Filth with an impressively solid backbone. To paraphrase Dick Emery, he is awful, but we do like him.