About ten years ago a notorious television programme was broadcast in many countries which purported to show secret film of an autopsy being carried out in 1947 on an alien found by the American Airforce at a so-called UFO site near Roswell, New Mexico. Its grainy badly focussed black and white look fuelled both its creepy cult status and the arguments of rationalists that it was a fake. Now here comes a film that claims to be the true, crazy story of that film. The two hoaxers Gary Shoefield and Ray Santilli are played by Anthony McPartland and Declan Donnelly, aka Ant & Dec [a boyish duo familiar to UK audiences as wacky but lovable comedians and presenters of game shows, who started out their careers as child actors in the children's serial Byker Grove, set in their native Newcastle].

Much has been made of this film being the opportunity for them to get back to their acting roots. But it just hasn't happened. It's not necessarily their fault, or down to lack of ability, in fact at times they both show they are capable of something deeper. But despite crowing that this is the chance for the pair to change direction and show us what they can do, the film-makers seem set on merely using their personas to carry the film, and it is written and shot precisely to that end. For one thing, if they're really actors, why not make them do the appropriate accents? Santilli and Shoefield sound like Londoners, and it's set in London, so why the Geordie thing? Could it be that the lads keep their accents because it's all part of the Ant'n'Dec package that will sell the film?

So, the two mates play two mates, Gary (Ant) a would-be successful, earnest worker in the offices of a biscuit factory, Ray (Dec) a wide boy who runs a stall selling illegal films he's copied on machines inside his Gran's cupboards. Beginning to sound like another famous Geordie twosome? - Yes, it's the Likely Lads all over again! Anyway, over to Cleveland USA they go in search of Elvis memorabilia, where Ray meets Harvey (Harry Dean Stanton), who has an interesting to film to sell... He was an airforce cameraman who was hauled out of bed one night in 1947 to film the alien autopsy, and still has the film. Ray sees it but we don't, is convinced it's genuine, and brings it back to Britain. But old films deteriorate when exposed to air, and when Ray tries to show it to Gary, lo & behold, it's almost faded away. Unfortunately he's already got a load of money to promote the film out of a pantomime drug baron by the name of Laszlo (aren't they always called Laszlo?), who will certainly have his guts for garters, or maybe his nuts for knick-knacks, if it doesn't come off.

This all takes quite a long time, too long, to establish, and involves some very cliched set-ups, in an old fashioned, cosy comedy mode. Obviously placed London buses, comic old people dating, chases, scary gay black bodyguards in posh houses, and much joshing between the two lads. Anyway, eventually we get to the best bit, the making of the hoax replacement film, which takes place in Gary's absent sister's front room. It's amazing what you can do with sausages, a lamb shank, a haggis, and a shop window model. This is quite funny, despite going with some ideas for too long - Geordie Gran gets her plate of biscuits in the camera shot once too often - and didn't you just know the sister would come back early? Still, it's well done, and fascinating for anyone with a memory of the original showing.

Afterwards, though, it's wearily downhill all the way. There's too much of the same, too much mugging and cheeky chappy doings, funny foreigners, the inevitable scene where the heroes joyously hurl their stash of ill-gotten dollar bills into the air, and a whiff of sex that predictably has Ant & Dec as love rivals to a glamorous blonde journalist - sorry, lads, you just don't look old enough! Ant & Dec do try to establish their characters, but the hoops they have to go through and the leaden writing lead to an awful kind of declamatory sit-com dialogue. Other good British actors do their best, Omid Djalili, Morwenna Banks, even an unrecognisable David Threlfall in frizzy red wig, but the only real pleasure to watch is Harry Dean Stanton, an oasis of calm in the midst of frantic madcappery. That mournful, world-weary face utters the truest line as he watches the hoax film - ‘What the fuck is that?' For a moment I though he was watching the same film as me.