Attack The Block (15)

Starring: John Boyega, Nick Frost, Jodie Whittaker, Luke Treadaway

The late-night mugging that sets up Joe Cornish's debut film is tricky to get over. It skews your judgement of the BMX-riding teenage hoodies (the film's "heroes") at the centre of the action. However, this alien invasion movie is still a whole lot more fun than most British action films, and is blessed with vigorous direction and performances, a zesty, dubstep-laced soundtrack from Basement Jaxx, as well as a perky script.

A stinky alien – who smells "like a shit did a shit" – plummets into Oval, South London, on Guy Fawkes Night, interrupting the mugging of a student nurse, Sam (Jodie Whittaker) by a five-strong gang. The hoodlums, led by flick-knife wielding Moses (John Boyega, in a very persuasive lead performance), track the alien down and slay it. "Welcome to London, brother," they inform its corpse, riffing on Independence Day. And, much like Simon Pegg's far superior Shaun of the Dead, there's a lot of riffing on classic sci-fi movies (ET, Predator, Critters) in Cornish's postmodern mash-up.

The teens, on Moses say-so (his catchphrase is "allow it"), take their intergalactic trophy back to their imposing tower block to show Nick Frost's podgy drug dealer/stoner, Ron (Frost plays Frost, just without his bosom buddy Simon Pegg to bounce off), debating about how much their prize will fetch on eBay and whether Simon Cowell could get involved – "Aliens Got Talent". Clearly, they're being too blasé about killing this smelly creature and before too long a motherload of beasties are unleashed from the skies – big black furry aliens with shiny teeth, they look like wild shaggy dogs/carpets. The invaders, in truth, don't appear to be terribly bright – you wonder how they had the nous to navigate their way through the universe – and they're not terribly scary. In fact, the drug lord, Hi-Hatz (Jumayn Hunter), of the estate where the youths live is far scarier, like a character lifted from The Wire.

The 42-year-old writer and director Joe Cornish, one half of The Adam and Joe Show and a comedian who appeared to make TV career out of Star Wars gags, invests Attack the Block with a lot of energy and has obviously tried to hard to capture South London "street" talk. Cornish doesn't, however, delve too deeply into the politics of gang culture or David Cameron's austerity Britain, although there's a nice, sly dig about Sam's worthy boyfriend volunteering to help children in Africa, rather than working with deprived kids right here in England. And Moses's exquisitely paranoid comment, "the government bred these creatures to kill black boys", is a political statement of sorts.

After an impressive chase sequence around the estate – the teens on bikes, pursued by the shaggy carpet creatures – Cornish's film loses some momentum and focus. We're not really given enough background details upfront. We need to know much earlier on, for instance, that 15-year-old Moses lives in squalor with a neglectful uncle, and still sleeps under his Spider-Man duvet. It would make us feel sympathetic towards this nocturnal pest. It's not enough that he's courageous, that he ends up saving Sam's life on several occasions, and that she eventually forgives him. We still need more and the attempts at high-minded seriousness ("actions have consequences", it dawns on Moses) towards the end feel too little, too late. However, the charm of the teenagers wins you over. Berating Sam with "you swear too much, man," is particularly witty, and Cornish's cinematographer, Thomas Townend, makes adroit use of the murky tower-block setting. As Moses would say, "allow it", and you might just ignore its many flaws and enjoy the full-hearted spectacle.