The saints and sinners of St Trinian's

Gabriella Wilde and Tamsin Egerton are the queen bees of St Trinian's, and there's quite a buzz about their post-school careers as well. Hermione Eyre joins the clique...

Tamsin Egerton and Gabriella Wilde throw their arms around each other, baby blonde tresses flying everywhere, long tapering arms flailing theatrically, a Ronald Searle cartoon come to life. 'Love yoooo!' Tamsin coos. 'Can we meet up soon and go shopping and have cake?' Filming St Trinian's 2: The Legend of Fritton's Gold, in Ealing Studios with Rupert Everett, Colin Firth and David Tennant, has clearly been a bonding experience. 'Actually, it was the most fun I've ever had in my life!' says Tamsin. The girls play Chelsea and Saffy, leaders of the school's so-called Posh Totty clique, roles they perform, it must be said, with some ease.

Both girls were born in Hampshire with rather grand surnames - and both have adopted stage names that are considerably snappier. Tamsin Egerton-Dick has given that annoying last part of her surname the chop, and Gabriella Anstruther-Gough-Calthorpe has discarded all three of hers to become Gabriella Wilde. After her precocious modelling career - starting at 14, she shot campaigns for Burberry, Pringle, Ghost, Topshop and Puma - she describes her new name as 'a new start' to launch, at the grand old age of 20, her second career, in acting.

She was picked up by Tavistock Wood, the agents who represent Dominic West, Edward Fox and Eva Green, after she approached Angharad Wood with a demo tape in which, rather resourcefully, she had recorded herself playing scenes from Emma. Saffy in St Trinian's was her first professional role, and to take it she ditched her degree in fine art at City & Guilds of London Art School. 'I'll always have art, but if I want to act I should do it now,' she says, clearly repeating advice she's been given. 'My agent helped me choose my new name after I was incredibly indecisive. I thought about using Gough, but that's quite difficult for Americans' - someone's clearly thinking Hollywood here - 'and then I hit on Wilde after talking through some old family names. No relation to Oscar Wilde, though, I wish!'

Her father, John Anstruther-Gough-Calthorpe, is a former chairman of the Watermark Group and the younger son of a baronet; her mother, born Vanessa Hubbard, was a model and sat for David Bailey and John Swannell. Two of her sisters, Isabella Calthorpe and Olivia Llewellyn, are actresses already (although both are half-sisters - Olivia from her mother's first marriage, to the late Sir Dai Llewellyn, and Isabella from her father's first marriage --Gabriella speaks of them simply as her sisters). Olivia, 29, had a small part in The Boat That Rocked, while Isabella, also 29, has appeared in How To Lose Friends & Alienate People and Richard Eyre's restoration drama Stage Beauty (fittingly, Charles II is a distant forebear).

She shone on stage at the National Theatre in The Voysey Inheritance, and playing Baby's older sister in Dirty Dancing in the West End, and is currently appearing in the university romp Trinity on ITV2, in a role, alas, best summed up as Posh Totty.

Gabriella says her sisters have warned her about 'the ups and downs' of the profession, and she won't be drawn on who her boyfriend is, possibly after her sister's experience of having her friendship with Prince William exaggerated all over the press (Isabella is, in fact, dating a different heir - Sam Branson). The sisters all have different agents, which keeps things from getting too incestuous. 'I'd really love to be a versatile, chameleon-like actress, stretching myself a bit,' says Gabriella, whose inspirations are Judi Dench and Kate Winslet. For now, though, swinging hockey sticks, invading boys' schools and sliding down fireman's poles in St Trinian's is a good start. The pole-sliding was all her own work, no stunt doubles required.

To get into character, she drew on memories of her schooldays at Heathfield boarding school in Ascot, where she learned the fine art of skipping prep, before graduating to vodka smuggling, which resulted in a suspension and a transfer to St Swithun's in Winchester. Was she bored or was she naughty? 'One made the other happen, probably,' she says airily. Her angelic appearance belies a certain wilfulness. 'I just didn't get on very well with school. My parents didn't mind. I think they were naughty themselves, and I was the last of a lot of children who had been naughty at school. Also, I had to own up about the vodka,' she says, darkly serious. 'Otherwise they were going to punish the whole year.'

Tamsin Egerton is, as a matter of record, 5ft 10½in tall, but nothing quite prepares you for her attenuated, almost alien presence. Drawn in long sweeping lines, her knowing eyes framed with spidery lashes, she's perfect casting as a St Trinian's belle. Her sexy, ditzy performance in the franchise's 2007 film kick-started her career, and soon she was everywhere: on the red carpet, on The Independent's short list of the world's most beautiful women (she came in at number ten), on the arm of Jenson Button --or so it was alleged, after they were spotted together at the Buddha Bar and at Bungalow 8 in the small hours.

There's definitely something of the It Girl about her as she sits down for the interview, complaining about jet lag (she's just got back from filming Noel 'Kidulthood' Clarke's 4.3.2.1 in New York) and wrapping herself in a cosy cardigan, a chunky Dior Christal watch dangling from her slender wrist. 'Isn't it bling? This' - she taps its encrusted surround - 'is black crystal and these are all real diamonds. They showed me loads and asked which one I wanted. It was the happiest day of my life. I was like, "I'd like the biggest!" '