The Hooray Henrys swishing their Union Jacks in the Albert Hall are in for a shock. And BBC One viewers switching on the Last Night of the Proms in the expectation of hearing singalong sea-shanties and easy-listening Elgar may be a bit bemused too. A 30-year-old Scottish composer with a penchant for raucous electronica has been commissioned to write a piece for the concert's supposedly populist second half. “Don't worry,” Anna Meredith laughs, “it's dead short - only five minutes. Over before you know it.” Then, slightly less reassuringly, she adds: “Like a punch in the face.”

Meredith's customary haunt is the Camberwell Composers Collective, which she runs in South London with four other young composers. They normally do experimental concerts to smallish audiences in jazz clubs. The Proms commission will be on a slightly larger scale. About 40 million people round the world will be watching and listening. And the piece involves 800 performers, including four orchestras and two choruses.

If you are wondering how that lot will cram on to the platform, the answer is that they won't. They will be spread around the country - in Glasgow, Belfast and Swansea, as well as in Hyde Park and at the Albert Hall itself. Everything will be held together by the miracles of satellite technology. Or perhaps not. There is the delicious possibility that it will all go horribly pear-shaped on the night. Especially as the 800 performers won't rehearse together until the day. “That's the really scary bit,” Meredith says.

Her original brief was to write a piece that used the BBC's various orchestras one at a time, and then simultaneously if possible. “That sounded straightforward enough,” Meredith says. “But then I realised that nothing could coincide. There are different kinds of broadcasting feeds going on. Some are on telly, some radio. There is the problem of feedback, of different satellites being used, and different delays for different places. And the sound that the musicians will be hearing through their monitors will come slightly before the sound that their audiences will hear. It's massively complicated.”

No question, then, of playing to a single unifying beat? “No,” Meredith says. “But all the conductors round the country will have headphones relaying what's happening in the Albert Hall, so at least they will be playing in the right place, even if they aren't exactly in the right time - if you see what I mean. I've tried to write rhythms that sound good whether they are in the correct part of the bar or not.”

Meredith admits that the piece won't be subtle. “I've gone for broad effects. And I guess that it's a wee bit irreverent, because that's the way I am. I also took account of it being in the second half of the Last Night, which is traditionally the time for Prommers to let their hair down.”

But what happens if the satellite links don't work? “Well,” she replies, “I've written the piece in such a way that the music in the hall can carry on regardless.”

Presumably the master score for this technological epic is as tall as a double-decker bus? “Actually, there's no master score,” Meredith says. “There are too many instruments to fit on a page. The conductors will have their particular orchestra's music written out in full, but only a few broad indications of what else is happening at the same time. That's even true of Roger Norrington, conducting the main performance in the Albert Hall. Nobody gets to see everything. I quite like that.”

Meredith grew up in Edinburgh and studied at York University and the Royal College of Music. Quirky and eclectic, she's no stranger to unusual commissions. Her other big work this year is a Concerto for Beatboxer and Orchestra. And with the writer Philip Ridley she is embarking on a youth opera for Aldeburgh next February that tells a grittily contemporary tale of doomed teenage love - with, she promises, “a huge climax in which quite a few characters die”.

But the piece for the Last Night of the Proms is, she admits, her most daunting challenge yet. Even choosing what to call the work gave her trouble. It was only last month that she came up with the distinctly odd title, froms. “And it's all in trendy lower case, I'm afraid.”

Er, froms? “It's a word I made up in the middle of the night, because in the piece I've tried to use a few of the characteristic elements from the Proms. So I was attempting to think of a title that could mean ‘from the Proms', and ‘froms' came to me in a flash. I think it's brilliant. No one else seems to agree.”

Is she worried about the public's reaction to the music itself? After all, the last time that the Last Night included an avant-garde piece - Harrison Birtwistle's Panic - the BBC switchboard was jammed with outraged viewers.

“I try not to worry about other people's reactions, because thinking about that sort of thing makes me write crap music,” Meredith replies. “Besides, with this piece I hope that the sheer visual spectacle will be gripping in itself. If people just roll with it, they will enjoy it.”

And how does she feel about taking a bow in front of 40 million people? “Oh, I've bought myself a hot new dress,” she grins. “But I expect I'll trip up and fall flat on my face.”

The Last Night of the Proms is live on BBC Two and BBC One this Saturday (8pm)

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