I'm glad I didn't see this in the cinema - I think I may even have screamed at one point!
The premise is as crumbly and hollow as the beastie-infested cave system, but you don't really care. Writer-Director Neil Marshall has gone one better than his Dog Soldiers, which had a lot of promise. This is very much the British horror classic that Creep might have been. For sheer terror it's nearly up there with a first viewing of Alien, though the cast of would-be cadavers were better defined in Ridley Scott's classic.
Marshall has described the movie as "Deliverance goes underground". Amongst other references there's a borrowing of Kubrik's famous helicopter tracking shot from The Shining and the use of a camcorder that reminded me of the The Blair Witch Project.