By Ian Youngs BBC News entertainment reporter

This year's Sundance film festival - the most prestigious annual showcase for independent movies - includes the world premiere of a documentary about the UK's legendary Glastonbury music festival.

The Glastonbury festival is having a year off in 2006

In June 1971, a teenager called Julien Temple ran away from school to join 12,000 free-spirited music fans at the Glastonbury Fayre on a Somerset farm.

The atmosphere "affected me deeply", says Temple, who remembers waking up at dawn to see David Bowie on stage.

"There was this sense of the festival being very much one connected event and everyone waking each other up and saying 'you've got to see this guy, he's amazing,'" Temple says. "And he was."

The festival has managed to survive the subsequent 35 years - going from strength to strength and, in the process, morphing from low-key hippy haven to mainstream national institution.

Last year's festival was the most popular yet, with fans snapping up 112,000 tickets in three hours and watching international stars on six main stages, surrounded by a ring of steel and tight security.

The Glastonbury festival has survived for 35 years

And Temple, now a renowned film director, has made a documentary about the festival's history. "I think it is an important English event," he says.

"I think it does bring together a certain kind of free-thinking version of Englishness that in a very interesting way mirrors the changes that have happened in the wider culture."

It is not always easy to notice the differences at the festival from one year to the next, he says.

But looking at its entire history, the way the event has changed highlights how the country as a whole has shifted - from style and music to attitudes and identity.

"When you look at it with the perspective of 35 years, you really do realise that not just the festival but the England we live in is another planet from what it was then, and we are very different people," he says.

"If people had told us in 1970 that we'd be sitting inside an iron-ringed fence with security cameras, we would have laughed them out of the door."

Hopefully you get the sense of surviving this event rather than being guided through by some narrator Director Julien Temple

Despite the obstacles that are charted in the film - from local opposition and travellers' riots to mud and gatecrashers - the festival has managed to retain some of its original spirit, Temple says.

"In a sense, it is a survival course, an outwards bounds course, and in a sense it is a kind of university for young people to find out about ways of living that they would never really be taught at school."

The biggest threat to the festival's spirit is the creeping advance of commercial interests, which is "a pretty difficult thing to control", he says.

"But if you do compare it to the other big festivals, it is still far less of a corporate advertising space than Reading or the other festivals.

"There is a will to keep it as something that isn't just there to sell stuff to people."

The film is intended to feel like "a weekend that lasts for 35 years and you're stumbling around through the mud and rain and sunshine", he says.

"Hopefully you get the sense of surviving this event rather than being guided through by some narrator - which is what [the festival] is like."

The festival attracted more than 112,000 fans in 2005

That has been achieved by using video filmed by fans throughout the festival's history as well as professional footage.

Mr Temple appealed for festival-goers to dig out old home video recordings a year ago - and received 900 hours of tape as a result.

"A lot of it was rubbish but I was very keen to try to tell it from the point of view of the audience and the crowd," he says.

"It is quite shocking how footage people film themselves has an intimacy and an immediacy and a truthfulness that a TV crew of four or five people sticking furry microphones into tents seem to just kill.

"It does help give that sense that you're watching it from the point of view of being there and being part of it rather than a long lens from the outside."