There’s nothing more old-fashioned than commissioning an artist to paint your portrait, right? Wrong, according to Jasmine Guinness, heiress, model, shopkeeper and muse to Natasha Law, on the eve of the painter’s new show. “It was a great sitting, really painless,” says Guinness, now in Honeyjam, her Portobello store.

“Natasha was one of the nicest people I’ve met. And she was quick.” Over to Law: “We’d wanted to do it for a while, but had both been having babies. We’d never actually met before, so it was great when it happened. I loved her face.”

A good bedside manner, brevity, conversation – to a portrait sitter, these are all good things, and possibly among the reasons why Law, sister of Jude, is increasingly in demand. “The portrait requests are up, to the extent that I worry if they’re going to interfere with my other work,” she says.

Dogs, children, pregnant ladies – Law has done them all, and she can add a special category all of her own. “I get asked for body parts, like a cropped nude,” she says. “A torso, perhaps. Or a bottom.”

You can see why when you peruse Law’s portraiture, which is like a graphic snapshot. Simply, she goes to the subject’s place, talks, does some line drawings and takes some photographs. “I like the idea of catching offhand moments. I don’t like stiff poses. It’s really important that they feel secure, particularly if there’s nudity,” she says. Then, back in her studio, Law paints them big with her signature household gloss. It’s only gone wrong once: “It was America. It was kids. They wanted a horrible interior-design job.” On happier occasions, she has liked the work so much, she hasn’t wanted to hand the pictures over. “It’s like having someone with me,” she says, “and it’s sad to see them go.”

This fresh approach is part of a new mood in personal portraiture, according to Sara Stewart of the portraiture agency Fine Art Commissions. “We’re seeing a real boom,” she says. “Portraiture is in fashion, and its new popularity is coming from private individuals, not companies.” Previously, the clientele had been “business types”, she continues. “Now, it’s all family portraits and decade birthday celebrations.” As with Law, getting the relationship right is vital. “Chemistry is really important,” she says. “Artist and sitter must get on.”

Portrait-painters recognise the power of a sitting. “It’s all about interacting with people, to get the depth and range of personality,” says Aldo Balding, whose commissions have included Gordon Brown. What about flattery? “I might portray some people in a positive light,” he says, “but honesty is the vogue.”

There may still be a place for the original purposes – such as legacy and immortality – of portraiture, but Guinness, who, as a stout heiress, comes from the world of bloodlines, says that depicting a mighty dynasty is far from her mind. “Absolutely not,” she shudders. “What a horrible idea.”

Room, an exhibition of works by Natasha Law, is taking place simultaneously at Eleven, 121 Charing Cross Road, WC2, and 11 Eccleston Street, SW1, from Thursday until February 16; www.elevenfineart.com

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