Stanley Johnson, the former Tory MEP, is a proud parent tonight, taking endless photographs as his progeny charms the audience. But it isn’t his son Boris getting all the applause up there, it’s the Mayor of London’s little sister Julia Johnson, singing with her band Second Person. Their music is influenced by trip hop, the sound of stoned Bristol, but this is Pizza on the Park, Belgravia, where the chief stimulant is a nicely chilled prosecco.

I ask Stanley if he wouldn’t prefer it if his daughter did something more respectable. “Well, it’s very nice to know that even my younger children are not making a total hash of things,” he says. “As far as I’m concerned, as a father, one cannot have a higher ambition for one’s children than that they shouldn’t make a total hash of things, and if I listen to this music it really grabs me. And she writes these songs — she writes the lyrics, she writes the music, and she sings wonderfully. In the past five days there have been 400,000 hits on YouTube for their song Wood. Actually, it’s 400,001, because I logged on to it.”

Up on stage, his daughter announces: “This next song’s called Come to Dust, which is the last line, as I’m sure you know, from a play called Cymbeline.” The crowd goes awkwardly silent. “Some of you must know it,” she wonders aloud. Yet more silence. “Um, Dad?”

It doesn’t always pay to be clever. A first-class degree in Classics might get you far in Tory politics or right-wing journalism, the two staple trades of the Johnson dynasty, but Julia’s literary credentials probably won’t help in her chosen field of pop music. Nor will coming from an upper-class family. At least, that used to be the case. But the music industry is changing. The current rise of the Tories has been accompanied by a growing acceptance of poshos in popular culture, and while the financial gap between the rich and poor might be growing, the cultures are mixing together in previously unimaginable ways.

Never is this clearer to me than a few days later, in a scuzzy old Soho punk venue called the 12 Bar Club. Half the audience is covered in tattoos and piercings, while the rest — the band’s close friends and family — are wearing corduroy jackets and nicely pressed summer linens. They have all come to see the debut gig of the New Forbidden, whose lead singer is Valentine Guinness, part of the brewing dynasty, while the guitar player is Loyd Grossman, the TV presenter and king of pasta sauce. If that wasn’t unlikely enough, the bassist is James Baring, whose family once owned London’s oldest merchant bank.

The performers are clearly having the time of their lives. Grossman’s smile remains beatific even when a joker in the audience tries to distract him from his guitar solo with the inevitable line, “Who would play in a band like this?” And both sides of the audience are captivated when Guinness rips off his shirt. Which does seems a waste of that rather lovely floral Liberty print.

But are they serious? A bunch of men on the wrong side of 40 and presumably as rich as Croesus — I mean, who would play in a place like this? Well, Grossman had a one-hit wonder punk band in the 1970s called Jet Bronx and the Forbidden, while Guinness led a series of indie groups in the 1980s and 1990s. The two are old friends who have wanted to make music together for years, and they say the time is now right. They have been rehearsing like mad and are prepared to do what it takes.

They admit that they didn’t have much musical success before, though. “The real barrier to success was the record companies,” Guinness says. “As soon as they found out what my surname was they ran a mile. I would keep it from them as long as I could and they actually liked the bands and the material — we had meetings, we got a long way with them — until they found out who I was. Then they always dropped me.”

He says that privilege used to carry a stigma, and people thought that music must be just a hobby for him, some kind of joke. “So I had this bizarre situation where I had a band that I had to keep secret from the press; the whole point of having a band is that you want as much press as possible! But if I didn’t I’d have the society diarist columns such as Nigel Dempster and William Hickey coming down and treating it like a circus act.”

He laughs. “I hope I don’t sound too bitter, I mean, it could also have been because I was a bit crap. But it’s different nowadays. It’s not that I think there’s a Zeitgeist situation whereby suddenly toffs are wanted by people; I don’t think there’s a massive new market for toffs. But I do think the barriers have broken down. The tyranny of the record companies is over — and those guys were often public schoolboys desperate not to get found out themselves.”

Thanks to MySpace and other internet sites, the father of two believes everyone can now find their niche. “I just feel so liberated by this. Now we are in a situation we haven’t seen since the 1960s, where, if you write good songs and perform them well, people are going to like you. They’re not going to come up with endless reasons why you’re not à la mode. It’s a level playing field.”

Indeed, it’s the internet that helped Julia Johnson’s band too — they might come from a moneyed background but, lacking a record deal, Second Person turned to the website Sellaband.com to raise funds and got £25,000 from fans around the world to record an album.

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