Twiggy was the face of 1966 and is shaping up to be the face of 2006. Hey, the way it’s going she might even be rear of the year, too. All she lacks so far to rejoin the modelling elite is the obligatory cocaine sting.

You may have thought that by the age of 56 Twiggy would have matured into Trunky, but try saying that in America where she has become a surprise “new” television star. Or in Marks & Spencer: the chain store looked more stuffed than a Christmas turkey until it persuaded the original supermodel to be seen in its threads. Now clothes fly out of its stores faster than Tony Blair capitulates in Brussels. This has prompted City boys to cry — for the first time in many a season — “buy shares in M&S”.

Sure, these days the stick-like one poses in woolly cardies rather than sheer hot pants. But 40 years on from her discovery, Lesley “Twiggy” Hornby is becoming Britain’s most important model all over again. Kate Moss may attract headlines about white lines but the model who shifts fashion lines is our Twigs.

The renaissance of both Twiggy and M&S symbolises a fashion transformation. Ever since the 15-year-old hairdressing assistant from Neasden exploded on Sixties London, fashion has been modelled by and for girls. But this ignored the vast mass of mature womanhood, whose idea of lingerie no longer stops at reinforced thermals. And it is dear old M&S that has spotted a hole in the market. Twiggy, meanwhile, has inspired mature fashion just as she once led young fashion. And suddenly wrinklies rock: Versace had Madonna and Demi Moore, Prada has signed up Kim Basinger.

That is far from ol’ Trunky’s only achievement of late. In America she has become a huge celebrity as judge of America’s Next Top Model, a kind of Pop Idol for would-be mannequins. Across America she is known as Simon Cowell with a hint of humanity. Not that life last week was looking terribly Hollywood for Twiggy: hello boys and girls, it’s panto time! Whenever a panto celebrity hears the children scream, “It’s behind you!”, the celeb must surely sigh: “Yeah, my career.”

I find Twiggy through the stage door of the Theatre Royal, Brighton, where she is starring — the lucky thing — in Jack and the Beanstalk. Well, I tell this most elegant of beanstalks emerging from the dressing room, you are certainly tall enough to play the giant. “I play the fairy,” she replies crisply.

Despite carefully applied baby blue make-up, lines are traceable around Twiggy’s eyes, but as she removes her coat at the Italian restaurant you see she is, if not twig-like, then scarcely more than a sapling. With her north London vowels, uncluttered by Atlanticisms, you expect her to be a jolly Babs Windsor sort, but she is sharp, focused, even prickly.

I ask what happened to the self-styled Justin de Villeneuve, her wideboy svengali who spotted her in that Neasden hair salon. “I have no idea and he was not my svengali,” she replies tartly. “He was my boyfriend who spent a lot of my money. He didn’t make me. It was his brother who called me Twiggy just to tease me. Justin came with me to my first photo shoot because my father, who was a very sensible man from Bolton, didn’t want me to go on my own. Anyway, I don’t want to talk about Justin. I thought we were going to talk about Marks.”

Well, do excuse me, Your Twiggness.

She, like the Beatles, the Mini and Mary Quant is an iconic figure of the Sixties yet it seems Twiggy did not swing. Or if she did, she is reluctant to talk about it. Instead she seeks recognition as a serious actress, even if she is strapping on fairy wings for eight-year-olds. Still, her ambition is not as outlandish as all that: she won two Golden Globes — best actress and best newcomer in Ken Russell’s The Boy Friend — when she sensationally quit modelling in 1970. She has enjoyed the odd success on Broadway and recently starred in BBC TV’s acclaimed contemporary Taming of the Shrew.

Alas, nobody is terribly interested in all that. As with old rock stars, the public want to hear tales of youthful excess; surely Twiggy must have had a Marianne Faithfull “Mars Bar” moment? “I was a baby; a very shy, unsophisticated 16-year-old who worked very hard,” she says. “I have never liked parties.” So the most coveted beauty in Britain would return home to Neasden after a day looking minxy for the cameras, and settle down for a nice spot of needlework. “I had a happy family but I was insecure in myself and hated the way I looked,” she says.

“I loved fashion and wanted to be a designer: I made my own clothes and had photos of Jean Shrimpton over my bedroom wall. I was the new look, but I certainly didn’t know it.”

Before her and Shrimpton, models were all “upper-middle-class girls looking for a husband”. Still, she soon felt she belonged: “Suddenly I was being flown first-class to New York, meeting amazing people; of course I loved it. I knew I was successful because I was on the cover of every magazine.” But almost as soon as she caught the camera’s gaze, she turned away.