That was how it was for Serena Mackesy. For Tony Blair - and thousands of other M People fans - it could be one night in Wembley. And tonight's the night.

Despite the fact that the point of pop music, like any other profit-making business, is to shift as many units as possible, the taint of commercialism is hard to scrub off. M People may have been championing the use of real instruments and performance through the "Who Sings? Who Cares?" dance scene years, but there is one word with which they remain synonymous: Peugeot.

Not that they mind that much. The James Terminator Cameron-directed ad that"Search for the Hero" accompanied (or was it the other way round?) was one of their smartest career moves. As Heather Small, the pint-sized soul diva whose voice has probably inspired more shouting on more dance floors than any other over the past six years, says, "It's the fact that the Peugeot ad was so successful that's the galling thing. But that's why you can't knock it."

"The first time it showed was during News at Ten on a Friday night, three minutes long," says Mike Pickering, the band's 41-year-old Svengali. "I kept thinking, `this is one of the best pop videos we've ever done'. We could never have got James Cameron for a video; they spent something like $3m. And then it got resurrected because the one with Kim Basinger was so awful; all the dealers kept telling them to put ours back on. We got a few holidays out of that."

Plus mega-sales, of course.Bizarre Fruit, the album from which "Hero" came, charted for two-and-half years. Fresco, released on 13 October, is selling like hot cakes; "Fantasy Island", the single released on Monday, notched up advance orders of 64,000, and tonight M People hit Wembley in an 18-date arena tour for which 200,000 tickets have been sold. The Blairs are going to Wembley, apparently, darlings.

They are unlikely to go away disappointed; M People know how to put on a good show, and the spread of their audiences is remarkable. At Cardiff on Monday, the 7,000-strong crowd ranged from school children to pensioners. Gaggles of twentysomething women wore their hair piled up in imitation of Small and men with trimmed beards and serious tonsures hugged plastic pint glasses to their chests. Everyone was there: cool kids and their grannies, people who dress to match their pit-bulls, wine bar devotees. Many had obviously never been near the dance scene from which the band originated, but they were dancing their support-hose off on Monday. Egged on by the performers. The four-piece - Pickering, Small, Shovell and classically trained guitarist/keyboardist Paul Heard - kept the house rocking with the help of seven other musicians. "One Night in Heaven" appeared sixth in line and the election night theme tune, "Moving On Up", closed the set. "Search for the Hero" reduced the matron behind me to screams of delight, while the entire audience sang along, each wrapped in a conviction that the lyrics applied uniquely to themselves.

Key to it all was Small, who, despite a reputation for shyness and an aversion to self-publicity, stalked the stage like a tiger in a burr-patch, while Pickering performed Supremes-like hand and hip movements by his microphone. Small is energetic, fierce and blessed with a voice straight from the darker recesses of the soul. Her volume, poise and control are hypnotic. Sod the Spicies; if you want your daughter to have a good role model, start here.

Now, 32, Small paid her dues with the rated soul combo Hot!House before becoming one of Mike's people (that's what the "M" stands for) in 1991. Pickering, 41, has a music business pedigree that's hard to knock - spinning dance discs at the Hacienda in the glory days of Manchester; heading house outfits Quando Quango and T-Coy; A&R-ing for Factory Records and producing the Happy Mondays. Hardly an appearance from nowhere. And they are adept at managing themselves, playing the market, catching the wave of New Labour optimism and riding it all the way to shore.

Touring, though, is both strain and pleasure. Small, particularly, suffers badly from nerves before a show: "Going out on stage takes up a lot of my day. I have a routine, and I have to take time to be by myself to gear myself up. You have to focus, because however I wake up at 8.30am, come showtime I have to transcend those feelings and enjoy the show. If it's a good show," she adds, "I get an adrenalin rush." Pickering laughs. "You're like a woman possessed when you get out there. Her muscles ripple. She frightens me." Small cackles; for a tiny women, her laugh could demolish walls.

She is vegan, teetotal, non-smoking - mostly for the sake of preserving that voice - and, with the exception of Shovell, who raises hell for all of them, all are more into their families than the party circuit. Pickering has a daughter at home and another on the way; James, Small's eight-month- old son by the Rugby star Shaun Edwards, is on tour with them. It's working out well. Small is amazingly cool about the whole thing, and Pickering is an enthusiastic extended family member. "It's great having James. It kind of makes up for missing my daughter, you know: you can have a great play with him. He gouges your eyes out and grabs on to your ears. He has got certain traits of his father you have to watch for. He'll be biting your nose off soon."

"I hate showbiz parties," says Pickering, "really hate them. I just think, well, I don't have to be your friend just because we're in the same business, and I find everyone is totally false at these parties, well, most people. We usually go and stand in a corner and have a laugh, and the odd person of the same nature will come and stand with us." They confess to enjoying the recent Downing Street party where they met Mike Leigh, a great hero, and got to see the rest of the house.

Heather generally sticks to home in Maida Vale, occasionally popping down to Ladbroke Grove, where she grew up: "They're really nice to me there. They've seen my career, and they're just glad that a local girl's made good. It's very, very busy, you know, but it's nice to go there and visit."

If there's one thing they hate more than showbiz parties, it's the critics. "Their attitude didn't really change when we got the Peugeot ad; they hated us all the time. There are exceptions, but the more popular we got the more they disliked us," says Pickering the next day in the hotel, hung over from post-gig partying but still fresh from a heavy workout in the gym. The band have been called manufactured, cynical, mainstream elevator music, "calculated soul charlatans".

"Musicians in Britain, every one of them, are all in despair about the critics. A lot of them don't even come and see us play live," says Small. This must be a sharp thorn in the side of someone who can match live version with the recorded one every day of the week. "Our inspiration comes from people like James Brown," says Pickering. "All the old soul, artists, the dance acts, used to play live; I used to go and see them all. At the moment there are a lot of people going around with DATs, [Digital Audio Tapes] even at lot of the big shows. Sometimes it's necessary, of course. The Spice Girls have a 40-track one at the back with all their vocals on, and they quickly fade them out so they can go `thank you'.

"I'd find it soul-destroying to go out there and know that night after night it's going to be exactly the same," Heather continues. "There can be times on stage when all the musicians are in synch, and you do something that is so magical, and the audience realise it, and you realise it, and you're all in on it, and that's what you aim for."