The Lady Vanishes: can you improve on Hitchcock? The plot concerns a truculent young socialite called Iris (originally played by Margaret Lockwood; here reprised by the excellent Tuppence Middleton) who is preparing to head back to London after a mountain holiday in the Balkans. After a bump on the head, she boards the train home in a state of befuddlement, meets a kindly old lady called Miss Froy - and then wakes from a doze to discover that Froy has vanished, her very existence denied by everyone else on board. So are the passengers engaged in a cover-up? Or is the heroine going mad? The resulting movie isn't so much a whodunit as a what-on-earth-happened, which Hitchcock played for laughs as well as thrills. And audiences loved it. The Lady Vanishes became the biggest British film ever made at the time, turning its cast into stars and propelling the director to Hollywood. Only the foolhardy, you might think, would try to replicate that in a BBC costume drama. Unless, of course, they were doing something subtly different with it... "Originally we thought it was just the Hitchcock film," explains producer Annie Tricklebank, who meets me on the steps of the ivy-strewn lobby. But when we started digging into it, we realised it was actually adapted from a book called The Wheel Spins, written, I think, in 1936. The author, crime writer Ethel Lina White, is now virtually out of print, her most successful novel forgotten. But Tricklebank adored it. "It's a complete delight and beautifully written," she says. So, rather than go back to the film, we decided to go to the book. To be purists, really. As a result, the BBC's adaptation, written by Fiona Seres, loses some of the movie's set pieces. Charters and Caldicott, for example, two terribly English cricket fans played by Naunton Wayne and Basil Radford, don't make the cut. "I love them, but we didn't think about putting them in ours," says Tricklebank. They were a Hitchcockian invention. It also meant a shift in style, cutting the humour and turning Iris's predicament into a genuinely tense psychodrama, which was White's original intention. "The thing is, it's meant to be a thriller," explains director Diarmuid Lawrence, who is a fan of Hitchcock, but not of his work on White's story. I'll be attacked by fans for saying it, but I don't think he had confidence in his tone in that movie. Take Iris's bump on the head, for instance. "In the film, Hitchcock has this rather risible shot of some hands pushing a flowerbox on to her head," says Lawrence, chuckling. It really doesn't compare with the shower sequence in Psycho. But there you are, you're told: it was an attack. The new version, he says, plays the same scene with a little more ambiguity. "You're left thinking, "Well, did someone hit her? Or is it all a fantasy in her head?"" Once everyone boards the train, the air of unreality increases. As does the claustrophobia. Lawrence deliberately fuelled this with the designs for his carriages, built in a Budapest studio. "We made our corridors just six inches wider than the real thing," he explains (filmset trains are generally greatly oversized). Technically that was very challenging, but also very satisfying, because the claustrophobia becomes part of the story. However, the budget didn't stretch to the £64,000 hire fee required for a gimbal, a tilting mechanism that sits under the carriage, allowing the train to rock authentically. Back in the lobby, Hawes demonstrates the ancient acting technique required in such circumstances. "We all had to do this," she says, placing a hand on the reception desk and tottering. It didn't always go smoothly. "You'd see people swaying, and then as soon as they had some dialogue they would sort of stop," she laughs. And there were these little dongle things that hang down to pull the blinds. You'd get all the way through a take and someone would go, "No one bashed the dongle!" to get that swaying too. And we'd have to go again. But it looked fantastic, with the extras all swaying in the corridors. It looked like we were really moving. It was so surreal, but I think it will be very effective on screen. BBC execs must now be hoping that the same will be true of their spruced-up Lady Vanishes. However the question remains as to whether fans of the original will approve. Lawrence thinks they should be prepared to give it a try. "For those who love the film, I think the thing to say is, "Come and watch this one'," he says simply. It's not a critique of Hitchcock's movie. It's different, and the tone is very different. There's no reason why you couldn't love them both. The Lady Vanishes is on BBC One at 9pm on Sunday 17 March