One doesn’t expect to find divine inspiration at Glastonbury. But on a Sunday morning the thoughts of the festival organiser Michael Eavis are closer to God than to grime. “We want the congregations in the village churches to hear something appropriate,” says Eavis, reacting to complaints from residents about ungodly language wafting from his stages.

ENO’s Ride of the Valkyries must have been a surprising call to worship in 2004. This year it falls to the National Youth Orchestra of Great Britain to wake up Worthy Farm’s 177,000 musical revellers – and appease the goodly folk of Somerset.

If any orchestra is a natural fit for Glastonbury, the uncrusty NYO is it. With players aged between 13 and 19, the band is certainly closer to its festival audience than your average professional orchestra. And one imagines their musical tastes run more easily from Purcell to the Pigeon Detectives.

“We are beautifully placed to take risks – in terms of presenting classical music to audiences – that professional orchestras aren’t,” says Jonathan Vaughan, the orchestra’s director. “The NYO is young and dynamic and it wants to challenge the orthodox.” Vaughan’s players are equally serious about promoting the music they play. “I’ve got friends going to Glastonbury, who’ve said: ‘Wow! You’re playing – we’ll come and see you,’ ” says the 16-year-old percussionist Katy Hebditch. “They’d never in their lives come to a classical concert.”

David Elliot, the 17-year-old principal percussionist, isn’t sure what to expect: “It’s a bizarre idea – an orchestra at Glastonbury.” But he is confident of one thing: “People are open to good music whatever the genre.”

Other orchestras have had a similar idea. Since an estimated 60,000 watched ENO’s Wagnerian wake-up call, competition is fierce to appear on the Pyramid stage.

“Michael Eavis told me: ‘I’ve got several orchestras knocking on the door; why you?’ ” says Vaughan of a courtship that began two years ago.

Vaughan played a simple trump card: the NYO’s prodigious talent. He invited Eavis to hear Britten’s War Requiem at St Paul’s Cathedral last year. “I have to be convinced,” says Eavis, “and that clinched it. The NYO kids are musically brilliant.”

Indeed they are, jaw-droppingly so. Technically excellent, the NYO play with a musical understanding and vitality that leave one happy to be alive. Four times as many hopeful musicians, all Grade 8 distinction or above, audition for the 160-strong orchestra each year.

Sir Simon Rattle, the trumpeter Alison Balsom and the composer Thomas Adès are among a juicy list of alumni.

Eavis is also keeping things local; a number of the orchestra’s members come from within 20 miles of Glastonbury, with some attending Wells Cathedral School – Eavis’s alma mater. The 10-year-old who listened to Radio Luxembourg on a transistor under his pillow has taken a keen interest in the NYO’s musical plans. Vaughan’s suggestion of Bernstein’s West Side Story Suite got the thumbs down. “Oh, God, no! Not film music,” groans Eavis. “Far too Radio 2. Can we have proper classical music, please?”

So Glasto will hear Copland’s Fanfare for the Common Man, John Adams’s Short Ride in a Fast Machine – whose metronomic beats will suit the dance crowd still going from Saturday night – and Dvorák’s New World Symphony.

Disappointingly, a performance with the disco duo Goldfrapp is off. Squeezing the band plus an orchestra of 90 players and their instruments on stage proved impossible.

“It would have been great to explore another rock or pop collaboration,” Vaughan says. Björk is on his wishlist, “but we had two weeks to think about a new piece.” Vaughan and the conductor Charles Hazlewood settled on Terry Riley’s In Cas sufficiently innovative. It is aleatoric; Riley wrote 53 phrases but how the orchestra plays them is up to each individual. “Putting classically trained musicians into an arena where they’re asked to make artistic decisions can be scary,” adds Vaughan, musing on the potential for a Spinal Tap-esque, free-form jazz exploration. “But the beauty of it pragmatically is that it can be as long or as short as you want.”

Artistic anxieties aside, getting the NYO to Glastonbury is a monumental headache. The festival falls smack in the middle of exams, so former members are being drafted in.

Camping is out of the question (“The risk assessment for that would have been off my computer,” Vaughan says). Instead the NYO will be holed up in the YMCA in Bath, which means leaving for the festival site by coach at 4.30am for the Sunday morning start. And come the inevitable downpour, everyone will have to change from wellies to shiny, black shoes – while avoiding the quagmire.

For the NYO, Glastonbury is one gig – albeit a pretty thrilling one – amid busy 60th-anniversary celebrations. They will play the very grown-up Salzburg Festival in 2009 and hope to work with the Royal Opera’s Antonio Pappano next year. But first comes the annual visit to classical music’s own summer festival, the Proms.

“That’s the biggie for me,” says Elliot. “We’re on beside professional orchestras. We’ are playing to the same audience, in the Royal Albert Hall. And we’re working with the best.” Sir Colin Davis conducted last year’s Prom. “This year it’s Mark Elder. I’ve heard he’s pretty good.”

It seems you can take the orchestra out of the concert hall, but not the concert hall out of the orchestra. “Glastonbury comes close,” admits Elliot, “but the Proms is the main event.”

*The NYO also play Prom 29 at the Albert Hall on August 4. Box office: 020-7589 8212

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