INTERACTIVE / Spawning a monster: The Aliens set has been recreated at the Trocadero. Dominic Cavendish prepares to hyperventilate

Down a dingy basement corridor in the Trocadero, Picadilly, a dozen men and women clad in combat gear are grimly clasping M41A pulse rifles. Whenever a shopper passes, they leap forward, all smiles, and hand out a leaflet. The sight is faintly ridiculous. But then the idea of people posing as space marines in order to lead members of the public through a mock-up of the Aliens set, while all hell breaks loose around them, sounds a tricky one to pull off.

Although only a few days away from its opening by veteran alienophobe, Sigourney Weaver, the Glaswegian creators of the Alien War experience, John Gorman and Gary Gillies, are quietly confident that they have hit on the future of family entertainment.

'There are some people who are too lacking in imagination to get their heads round this,' muses Gorman, 28, a former freelance video maker, as he surveys one of the countless iron walkways that criss-cross the 12,000 square foot site. He teamed up with sound mixer Gillies, 37, after they found they had a mutual admiration for the films' visuals. 'As a 15 year old I saw the original Alien underage and the special effects blew me away,' he explains.

His fascination led him to start collecting the production costumes. Like the alien itself the idea took shape over a period of time. The first thought, four years ago, was to put on a theatrical aliens show. The end result is still, like the alien, incredibly simple. 'Basically the alien is just a reproductive organ, intent only on breeding - like a walking penis. Alien War works by just scaring the wits out of people.'

Flashing strobes and dry ice went down well in Glasgow. Over 100,000 people experienced Alien War there last year, several hyperventilated and quite a few fainted. With over pounds 1m invested in the technology, which allows each rifle pulse to trigger 275 watts of sound from concealed speakers, the two believe Alien War far outshines traditional exhibitions, laserquest games or even virtual reality. 'What is the point of virtual reality?' asks Gorman. 'It's not really happening. People will prefer this to passive entertainment. Once you're inside you completely believe the scenario. After all no one knows whether or not aliens exist. We're letting the public use their own imagination.'

Imagination is what is required above all from the space cadets. Not only do they have to ad-lib as occasion demands but they are not even being allowed a peek at an alien (in or out of its pod) until the show's opening. Like several other marines standing opposite the new Space Snack Bar, 29-year-old Maria Bergman from Sweden hopes Alien War will be her big break as an actress. Head wrapped in red bandana a la Ripley she confides: 'Sigourney Weaver has always been a big hero for me. Ripley is such a strong character part for women and you don't get many of those.'

The most successful ingredient from the movies and that most obviously missing from Alien War is Sigourney herself. But perhaps it will be the prospect of spotting Ripley clones rather than the hollow threat of being torn limb from limb which will prove the real crowd-puller.

From today, The Trocadero, 13 Coventry St, London W1, 10.30am-12m'nt daily, adults pounds 6.95, children pounds 4.95

(Photograph omitted)