On Shaftesbury Avenue, central London, on the broad pavement outside the NatWest bank, the artists arrive soon after closing time and set up for the night's work. They are a mixed bunch. Some produce crude caricatures, with outsized heads on perky, miniature bodies. Others take the task more seriously, presenting themselves as maestros whose work just happens to be inexpensive. For as little as £20, they promise, and in less than an hour, they will sketch or paint your portrait.
Long intrigued by these people, I recently took a seat in front of one of them, who identified himself only as Billy. He clipped a sheet of A2 paper to a board and set his box of paints and brushes on his lap. Over the following 40 minutes, he worked effortfully, tutting, frowning and biting his lower lip as he furiously wiggled brush against paper.
Billy had his back to the passing tourists and office workers, many of whom stopped by to stare over his shoulder. Although predictable, this was not something I'd actually foreseen. It was bad enough submitting myself to the penetrating gaze of the artist. Raised eyebrows and wrinkled noses from utter strangers was another thing altogether.
What made me try it? Well, after many years in which portraiture was seen as less accurate than photography and less interesting than abstract art, it seems to be coming back into fashion. Just at the end of last month, for instance, a portrait of the Prime Minister was unveiled at Portcullis House, the relatively new parliamentary annexe on Westminster Bridge. Blair has long resisted having his portrait painted - perhaps because it seemed grand and old-fashioned, and incompatible with inviting the likes of Noel Gallagher to No 10 for drinks. But now he has acknowledged that all prime ministers must pass through this rite, like it or not, just as they must submit, every so often, to the humiliation of canvassing for votes.
Interestingly, this first portrait for which Blair consented to sit was the work of Jonathan Yeo, son of the Tory shadow culture secretary, Tim. The artist has a theory about why modern politicians need portraits. "We know what they look like," he says. "But politicians tend to be portrayed either in slightly bland, manipulated images or in savage, exaggerated cartoons: one is bland homogeneity, the other brilliant caricature." Portraiture, ideally, falls somewhere between the two. (The result is not always satisfactory, however. Churchill disliked one portrait so much that he burnt it.)
The idea of commissioning the Blair picture - along with portraits of William Hague and Charles Kennedy - was conceived by Tony Banks, chairman of the parliamentary works of art committee. "Britain has often used war artists," Banks said. "We decided to have an election artist."
As this suggests, and in keeping with long-standing tradition, the subjects of portraits have usually been men whose achievements were deemed worthy of public commemoration. These days, the clients remain predominantly male, but the sitters are as likely to be women, or children. According to Charles Saumarez Smith, the director of the National Portrait Gallery, "commissioning a portrait is now seen as an affordable luxury".
Partly responsible for this state of affairs is Sara Stewart, who established Fine Art Commissions four years ago to put clients in touch with artists (of whom she currently has more than 85 on her books). Clients come into her office and leaf through portfolios, looking for an artist whose technique particularly appeals. But there are other factors to consider. Men are generally better at painting women, Stewart believes, because "there is some element of flirtation" involved in making a portrait. For similar reasons, she would not send a 70-year-old artist to paint a five-year-old. "They won't have the energy or rapport."
Having chosen an artist, clients talk with Stewart about details such as the number of sittings required and price. Many ask if the picture can be changed if they don't like the final result. "But that rarely happens," Stewart says.
One client, Louisa Uloth, got herself painted by Nick Bashall as a present for her husband. Altogether, the sitting took 70 hours, over a period of eight weeks. Uloth says getting the portrait done was not egocentric: "I don't think of it as a picture of me - it's a girl in a white dress. And I can live with that very happily."
Uloth's portrait was painted in the style of John Singer Sargent, but she wore a wedding outfit designed by Vivienne Westwood. Similarly, Yeo's portrait of Blair effected a modern twist on an old-fashioned form by showing him in shirtsleeves - the first time a prime minister has been painted like that.
But some portrait painters think overly modern appearances may later be regretted. One of Yeo's friends, Maria-Teresa Meloni, an Italian artist and book illustrator based in New York and London, has painted portraits for people from the worlds of finance and high fashion. She tries to dress her subjects in clothes that will not come to be dated. One recent portrait, for example, showed a boy in a collarless shirt. "That look could be from a hundred years ago," says Meloni. For another, she dressed a girl in an antique lace frock she had found in Greenwich Village.
Meloni believes the motive for getting a portrait done is simple: people have an appetite for classical beauty and they also like the idea of immortality. "People want portraits because they want to present beauty to posterity," she says.
Her portraits in oils can cost anything upwards of £10,000. Smaller pictures, just four inches square, cost £2,000, as do the works in gesso. Her clients, who have recently included Tamara Mellon, the managing director of the shoemakers Jimmy Choo, and stalwarts of Deutsche Bank and J P Morgan, are not the sort to be fazed by such prices. Some don't even want to know the cost until they have already decided to go ahead.
"You don't sell with price," Meloni explains. Nor with the promise of a quick fix: "I have to be granted at least one solid day . . . The fastest I can do this is two weeks, for a gesso."
Yeo, for his part, had five weeks to work on his election portraits. He saw the politicians in public contexts, but also in private. "It took a while for them to trust me, but eventually they forgot that I was around." Blair was the hardest to work with, Yeo says, because he seemed unable to relax.
"Being painted is very much an experience," says Sara Stewart. "If you can relax and go with it, that's when you get a really great painting."
On a tight budget, that is not easy. On Shaftesbury Avenue, my discomfort was compounded by Billy's evident dislike of his fellow painters, unrelated to him except by proximity. They included several aggressive salesmen, including a particularly vigorous self-promoter who popped out of his seat whenever passers-by paused, even briefly, to peer over Billy's shoulder; he fanned before them a book of postcard-sized reproductions and shouted prices that, astonishingly, failed to impress.
Another pushy type, whose canvas stood next to Billy's, was lucky enough to find a woman who, though reluctant to do business with him, had a young son eager to be pictured. She consented, eventually, only because the artist promised to finish with plenty of time for them to get to the show at a nearby theatre before the curtain rose. But as the minutes passed, it became clear that this deadline was impossibly tight.
Sitting in my backless chair, frozen into the position I had adopted when I first sat down, I was able to study the mother's expression in detail. She stared at the developing image of her son with a polite smile that gradually faded to a half-smile and then lesser fractions of pleasure. With two minutes to go before the play started, she snatched the boy from his seat and told the artist she was sorry, but she had to go and would not be able to pay for his unfinished masterpiece.
My own portrait, when Billy eventually showed it to me, turned out to be frankly hideous. The eyes were good, the hair OK, but the smile resembled the leer of a pervert.
Conscious that the crowd was watching this moment of truth, I dissembled, smiling as Billy rolled up my picture, and still smiling as I handed over his modest fee. But after I had got a few yards up the road, I abandoned the fake cheer. This portrait didn't make me look like a million dollars. Nor did it look like what I see in the mirror. Still, was it possible, I worried, that it looked like me?