If you're a fan of British horror films, or the horror genre or just provocative British cinema generally and you happen to live or work near a cinema showing Eden Lake, see it as soon as you possibly can. Should locality be an issue, take a bus. Hitchhike. Run. You won't thank me for it but it needs to be experienced. Because Eden Lake is the most powerful and illuminating British horror movie since the turn of the millennium.

For a so-called 'hoodie horror', it also pulls off the near-miraculous trick of avoiding 'Daily Mail'-style editorialising. (For starters, none of these little lords of the flies even wear hooded tops.) It is also deeply distressing, tremendously upsetting and not very nice. At the public screening I attended, when the lights came up, the audience just stared at one another with hurt "Oh. My. God" expressions, before limping silently out of the cinema. Incidentally, a tip: don't take loads of cinema food in. You won't eat it. Appropriately, I accidentally kicked my mostly untouched popcorn all over the steps as the credits rolled. Then hastily tried to scoop it all back into the box, to reassure the shellshocked couple next to me I wasn't a lout.

But the film is instructive. Even if the only lesson to pick up is to mind your own business and don't take on a pack of stabby kids. They are less socialised than you and I, have faster reaction speeds, and they will hurt you. My policeman friend recommends you try and get in a few punches if you can, as those who helpfully assume the foetal position are just asking for trouble.

Eden Lake may be based on several real-life cases or none whatsoever (actually, my copper mate does recognise a couple of incidents from the film as being grounded in reality; one not so long ago in which a bunch of adults were roasted to death on a bonfire by a pack of kids). But it's that feeling of utter authenticity or likelihood that gives Eden Lake its terrible power. Unlike The Descent or 28 Days Later, this is not "just a movie"; we do not emerge from the ghost-train ride blinking in the sun, laughing off those rubber monsters. The stats don't lie. The events depicted in this film are barely, if at all, exaggerated. Our children are literally becoming the death of us.

The dread is only heightened by our identification with the protagonists, whose sense of right and wrong is, though flexible, at least ingrained. We watch with splayed fingers as the man upbraids his teenage adversaries; not because we think what he is doing is wrong, but because we know it is right. He is on the side of light. And society is made craven and pathetic because, unlike him, it will not stand up.

About five years ago, I did take on a pair of physically aggressive teens in a train carriage, one of whom threatened me with a knife, and managed to restrain them. Would I have tried that these days, in 2008? I might think twice. Though hardly appealing to our better natures, Eden Lake forces us to confront ourselves. And above all, that is probably the most disturbing thing about this haunting, numbing movie. You will not be able to get it out of your head for days.