Sigur Rós play Festival - watch an exclusive video EMI

Sigur Rós are one of those bands that provoke long, sombre, painfully poetic tributes from people who have either forgotten – or would rather avoid – the truth that they are four blokes in a pop group who like getting drunk and making a racket. When I mention to the gently hungover guitarist and singer Jónsi Birgisson a quote from an American magazine that described how his "angelic falsetto coasts like a hang-glider riding the breeze to the edge of the sea", he and bass player Goggi Hólm almost fall off the sofa laughing.

"I just don't understand why anyone would even write that," Birgisson says, noisily stirring a cappuccino.

"This person should hear our sound-checks," Hólm nods. "They are fucking crazy. We just play all these big heavy metal songs – nothing is played straight." There are no pellucid, glacial towers of crystalline something or other then, I ask?

"No!" yells Birgisson. "And no golden tears falling from heaven, either. It used to be difficult for us, five interviews a day full of these very intense discussions about frozen landscapes and all of that, and we can't explain or describe the music any better than anyone else; it's really only the feeling that comes from it in the end that matters."

Hólm leans in. "Sometimes during those interviews I'd sit there and think: 'What the fuck has happened to my life?'"

Birgisson and Hólm are in London to talk about Inni, their new concert film and double-live album. The film, directed by Vincent Morisset, takes the concept of up-close-and-personal and makes it all a bit more up close and personal. Shot in black and white, with almost no audience footage, the film is a multi-camera homage to a band that, far from freewheeling around on thermals, clearly have to work hard to create their music, despite it sounding as if it just falls out of their fingers.

When I ask them how they prepare for such a show, Birgisson says: "Drinking a lot helps." Hólm thinks it's even simpler. "You just turn it on," he says. "It's like pushing a button. You walk on and do it."

So instruments are hit and stroked and kicked and bowed and tapped and rattled, but they're all so busy there's no time for anyone to quickly place a foot on the monitor and "gun down" the audience with their guitar, or cup their hand to their ear and provoke a rowdy singalong.

"We never even throw a pick into the audience," Birgisson admits. "We're not quite Iron Maiden yet. We're saving all that stuff for our retirement shows."

"When we play together we're in our own space," Hólm says. "We zone out from each other completely. Our music is all about the very tiniest, most minute details and they must be played exactly right or it will sound different, wrong."

Each of the limited, special-edition box sets of Inni will include, among other things, light-sensitive paper, etched 7in singles, an exclusive short film called Klippa and what's described as a "unique concert artefact", which I'm hoping doesn't mean old bits of gaffer tape pulled from the stage floor. "Oh no," Hólm says. "It's sweaty socks! It's like, we're done with all this, you can have it."

"We're moving on," Birgisson says. "In Klippa you see them cutting up our old costumes into little bits, and they're what's included in the box."

"All totally unwashed," Hólm laughs. "Three-year-old sweat sealed in a bag!"

The band's last film, 2007's Heima (Icelandic for "homeland"), showed them on tour in Iceland and attempted to explain who these people are and the place they all come from, but the live footage was hampered by what they describe as the "really strange venues" they chose to film in. The performance of one song, Gítardjamm, was captured inside an abandoned herring oil tank, while the idea of a record to go with the film was rejected outright.

"Live records seemed pointless to me," Birgisson says. "We already had really good studio recordings and really good bootlegs. But making this film made us certain you had to be able to just listen, too. Inni is a document of what we took years and years creating."

He goes on to describe the film as being "a straight-up rock show, with a twist", which would make a startling tagline for any future concert posters. It was meant to be finished and out at least 18 months ago, but no one liked the first cut.

"It was completely clean and boring and not how we imagined it all," Hólm says.

"We all hated it," Birgisson says. "And we were pretty brutal about it. But we felt like Vincent explained it one way and it turned out another. We all have such strong visions, we imagined something totally different. We've been playing together for so long, we don't have to discuss stuff, we just know."

The band were about to pull the plug on the project when Morisset showed up with a re-edited clip of Ny Batteri that he'd transferred from digital HD to 16mm film and reprojected and refilmed through, among other things, glass.

"It looked shitty and scratchy," Birgisson says enthusiastically. "And I mean that as a compliment, it was a huge improvement. Our problem has always been that people are too respectful of our work. They don't want to go too far and we hate that. We'd much prefer them to fuck it all up, to go fucking crazy. I mean, I'd never watch a live concert movie. They're boring; all the same shit you've seen before. It's hard to make it interesting."

Hólm adds: "This one has ended up being something we're all fairly happy with. That's as good as it gets with us. With the music we don't release anything unless we know it's really good, but a film is someone else's vision."

"It's their creation," Birgisson nods. "But, of course, we do have a say. We've been together 17 years in January, so we should do." Goggi looks up. "Seventeen years? I can't quite believe that…"

Birgisson had been in two grunge bands, Stoned and the talent show-winning Bee Spiders, before forming Sigur Rós – named after his baby sister, Sigurros (Victory Rose) – in Reykjavik in 1994. Two years later they released their debut album Von on Smekkleysa, a local label, whose catalogue included punk and pop artists alongside a large amount of jazz and classical recordings, but it was 1999's Ágætis Byrjun that pushed them over the edge. Brad Pitt, Madonna and David Bowie began to rave about them in interviews. Radiohead recognised them as kindred spirits and adopted them in rather the same way the Stone Roses once did with Oasis, only with less lager and drugs. Marketing types begged them to place their music on adverts, but they always declined (their website includes a wonderful section featuring the resulting attempts to mimic their sound). The band's ascent to the art-rock superleague was confirmed when Tom Cruise placed his toothy seal of approval on them, welcoming the band's Smashing Pumpkins meets Procol Harum meets the Orb masterpiece Svefn-g-Englar on to the soundtrack of his 2001 film Vanilla Sky.

"I remember the party for that film," Hólm laughs. "That whole scene was so plastic. I was outside drinking a beer and a guy walked up to me and talked about himself for five minutes, then asked what I did. When I told him he just walked off."

"It's funny to see a big star like Tom Cruise up close," Birgisson says. "He's treated like royalty. He arrived with Penelope Cruz and they're both really, really small. I watched them walk through the room surrounded by bodyguards and even though it was an industry event, people were screaming and trying to touch them."

Did you meet him?

"Oh yes," Birgisson says. "It was like being introduced to the queen."

"A woman actually tried to drag me away so she could get closer to him," Hólm recalls. "All that is so far away from our real lives. When you fly home and walk the dog or dig the snow out your driveway, it feels kind of weird. I couldn't live in LA. I'd be found dead, face down in a swimming pool with a straw sticking out of my nose within a month."

"But it's fun to visit," Birgisson smiles.

Meanwhile, somewhere in Reykjavik, in a studio decorated with fan art (their favourite piece depicts a person floating in space, "half man, half woman: one saggy bag up here, one ball down there and an umbilical cord. It has the moon and sun in it"), Sigur Rós are slowly working on a new record. Inni marks the end of the line for a lot of Sigur Rós material. There is one last album left on their contract with EMI/Parlophone (this record is coming out in the UK on their own label, Krunk). And after that?

"Well, we won't be the same, we're heading in a new direction. The music you hear in that film will never be recreated, Hólm says."

"The point is, like you said, we're happy to be seen as four dudes in a band," Birgisson adds. "Sigur Rós has never been about something specific, we just don't want to stop. We just want to be true to ourselves."

Inni is out now on Krunk.