We are peering through a thicket of stalagmites into a cavern full of carnage. In front of us are the ravaged corpses of a stag and a large dog. The ground is strewn with blood, viscera and what appears to be a spinal cord, species undetermined. And in the centre of the cavern three battered-looking women are preparing to fight for their lives against a pack of naked white humanoid creatures with bat-like ears. “Nick, can you get into your throttling position on Natalie?” says the stunt co-ordinator to one of the creatures. “Lots of gnashing and screeching please,” reminds the first assistant director.

“And, action!” All hell breaks loose: clawed hands grab female ankles, Lycra-clad knees plunge into slimy stomachs and rubber torches are smacked into fanged mouths. “Cut!” comes the yell. “Let's go again. And this time I want even more nerve-shredding desperation.”

It's safe to assume that the makers of The Descent 2 aren't departing from the formula that made its predecessor one of the surprise horror hits of 2005, grossing more than $57 million from a budget of $3.5 million. A tale of six women whose caving trip in the Appalachian Mountains is rather spoilt by hordes of subterranean “crawlers”, it was claustrophobic British terror at its best.

In the sequel, Sarah (Shauna Macdonald), the sole survivor, returns to the scene of the slaughter with reinforcements. The plan was always to retain the style of the original, whispers producer Christian Colson as the crew prepare for another take: “Often sequels modulate from being horror movies into action movies but we wanted to keep the same tone: a low-tech horror movie that scares the s*** out of you.”

We're nowhere near the Appalachians, of course. Neil Marshall, who wrote and directed the first film and is making a rare visit to the set in his role as executive producer, realised long ago that “to shoot in a real cave would have been impossible and probably deadly”. The venue instead is Ealing Studios in West London, where the cavern has been constructed from insulation foam and principal photography is almost complete.

Outside in the June sunshine is a plaque bearing the names of the Ealing comedies that were shot here: Kind Hearts and Coronets, The Ladykillers, The Lavender Hill Mob. Inside, muscle-bound crawlers huddle by a heater. “It gets really cold,” one explains. “Especially once they have applied the KY Jelly.” They are basically naked, with just white body paint and codpieces to preserve modesty, plus a coating of KY for that slimy look. One is having his chin re-attached by a make-up woman: “I was eating that dead dog rather too enthusiastically.” Alec Guinness must be spinning in his grave.

Having come up with the unprecedented concept of “an action-led horror film set in a cave with a female lead cast”, Marshall opted to hand the directing to his editor, Jon Harris, for the sequel. “He was the natural choice because he knows the world better than anybody except me,” Marshall says. But with the rest of the creative team returning, he admits to a twinge of regret: “It's like all my gang have gone out to play and I can't go with them.”

Playing they certainly are. Next door, James Watkins, the writer and second unit director, is shooting a scene with Macdonald's stand-in. “She crawls down a tunnel with a crawler at the other end and nowhere to go,” he explains. “It's all about claustrophobia.”

Farther down the corridor, the production designer Simon Bowles shows us his concept models. There's a miniature version of the feeding cavern we have just seen, where the crawlers consume the prey they hunt on the surface, and a model of the mine complex through which our heroes enter the crawlers' lair.

In another corner is a scene that will be familiar to fans of the first film: the corpse of Sam, one of Sarah's former companions, hanging from a rope above a chasm. Sam turns out to be useful in the sequel. “They realise that the best way to get across is to jump on to her lifeless body and swing across. Because she's covered in congealed intestines they start to slip off, so they have to put their hand in her ribcage and pull themselves up.” The aptly named Bowles beams like a new father: “It's a special moment.”

For all that, Colson insists this is a more optimistic film. “We left them in a dark place last time. This moves towards atonement, redemption. It's more of an ascent than a descent.”

Tell that to Macdonald, who has spent five weeks wrestling with crawlers, and squeezing through tunnels, often in darkness, sometimes underwater. “How could I do this again?” she smiles, proffering a hand covered in congealed gore (“It's not meant to stain”). She had nightmares on the first shoot: “Blood, guts, Armageddon, diseases, murder. Every situation you don't want to be in, I dreamt about.”

But, despite having also had a baby since then, she didn't take much convincing to reprise her role. “The crawlers come back with a vengeance,” she says. “We've got monsters with even more moves.” Plus bulges in all the right places? “My goodness!” she giggles. “You kind of forget that they're real people sometimes and just stare at their bottoms.”

So who needs Kind Hearts and Coronets? The future, clearly, is blood, guts and derrieres.

Descent 2 is released early next year

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