Perhaps the best thing to happen to British cinema over the last couple of years is the way in which James McAvoy seems to have replaced Ewan McGregor. Here McAvoy is Bruce Robertson, not so much a maverick cop as a sadistic psychopath. He cheats, lies and snorts his way toward promotion while his various demons play havoc with his tormented psyche. Adapted from an Irvine Welsh novel, Filth is set in Edinburgh and like Trainspotting presents the darker underbelly of the city. Think of it as Moonshine on Leith.

McAvoy deserves an Oscar for his performance here. Obviously he won’t even get a nomination because the film is so brash, irreverent and grim. They’ll probably give one to Ewan McGregor instead. But McAvoy’s Bruce is so monstrous and yet so human – a duplicitous, destructive maniac who carries impossible pain in his blue eyes. McAvoy’s body constantly contorts to realise this twisted creep of a man, who makes Dirty Harry look like DI Joseph Chandler. Bruce Robertson is a sex-obsessed, drug-addled, borderline-rapist, alcohol-fuelled, back-stabbing, dirty-secret of the polis – bad hair, bad skin, bad person. Between this and Danny Boyle’s Trance, McAvoy proves himself an Oscar-worthy leading man, more than capable of balancing black humour, profound sadness and the very darkest aspects of humanity.

Filth might not be Trainspotting, but then what is? Danny Boyle’s discipline and flair for music are notably absent, but Filth chooses wisely to avoid aping Trainspotting as last year’s Ecstasy foolishly did. More so than Trainspotting, it seems influenced by David Fincher’s Fight Club – a dark, surreal and often funny exploration of human psychosis, with penises. The marketing for Filth presents it as much more of a comedy than it really is. There are certainly funny moments but Filth is far from a comedy; it’s a harrowing character-study, and as a representation of mental illness it is to be commended its sensitivity and boldness.

With its sex, drugs and political incorrectness, Filth won’t be to everyone’s taste. Many will find Bruce Robertson hard to spend any time with, and the creepy moments of surrealism will put people off. But if a feature-length Prodigy video sounds like your thing then you’re in for a cocaine-fuelled rollercoaster ride. Eddie Marsan and Jim Broadbent provide superb support but this is James McAvoy’s moment. Filth is sordid, strange and subversive, with one of the year’s best performances, endings and posters (above). On top of all that, it’s a Christmas film – ho ho hoe.