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On the day the Russian president, Dmitry Medvedev, said to the West, “We are not afraid of anything, including the prospect of a new cold war”, the Icelandic four-piece Sigur Ros took to the stage in St Petersburg. In an austerely beautiful palace of culture, where once the nomenklaturawould have greeted the latest grain figures with synchronised clapping, the band’s openly gay, Gormenghastian lead singer, Jonsi Birgisson, stops sawing at his guitar with a cello bow and invites members of the audience to rise from their seats and dance. The suggestion does not go down at all well with the hostile, preperestroika-style security guards. One in particular seems especially irked. When a heartshaped red balloon slips its owner’s grasp and floats serenely up towards the top of the proscenium arch, the crowd watch it with smiles on their faces. He, on the other hand, follows its trajectory with tiny, angry eyes, and shortly afterwards coordinates a show of muscle that forces people back into their seats.

Outside on the streets, summer has disappeared: the temperature is chilly, the rain torrential. On the way to the soundcheck, the tour bus is stopped by the cops. It is that sort of day. Inside the venue, hatchet-faced harridans check tickets and treat requests for beer with surly contempt. By a beautiful irony, Sigur Ros are performing songs drawn chiefly from Meo suo I eyrum vid spilum endalaust (With a Buzz in Our Ears We Play Endlessly), their liveliest, most joyous and propulsive album yet. Once upon a time, back when they made records so ambient and open-ended, they might have been designed to send the masses into contented catatonia, Sigur Ros would have been music to the security guard’s ears. Now, though, things have changed. Their recent single Gobble-digook, and their new one, Inni mer syngur vitleysingur (Within Me a Lunatic Sings), defy you not to dance. So we do, but only briefly. Bewildered by the mostly muted reaction of the crowd, the group curtail their set list. They aren’t playing endlessly tonight.

“That audience,” Birgisson says later, safely back in the hotel bar, “was very weird.” Gazing at the cocktail menu and toying with a posh playgirl or an uncle Vanya, the singer contrasts tonight’s response with the show the band played at this summer’s Latitude festival. On that occasion, despite being viewed as unlikely headliners, Sigur Ros drew the biggest crowd and carried the weekend with a spellbinding, exhilarating set, Birgisson’s haunting falsetto floating out into the Suffolk night.

Their fifth studio album is the first the band have made away from the comfort zone of their own recording space in Reykjavik, and it shows; they travelled instead to Cuba, New York and London, hiring a 67-piece orchestra and a huge boys’ choir, and even singing one song in English (previous albums have been in either Icelandic or a made-up language known as Hope-landish). The record was finished last May, mixed in less than a month, then speedily released. Its brio, brevity - Sigur Ros song times have been known to spread themselves a bit in the past, to put it mildly - and skittishness astonished their fans, ravishing some, deterring others.

Georg Holm, the band’s affable bassist, is wary of the latter, flame-keeping constituency. “Sometimes,” he says, munching on the caviar the group’s publicist had expressly asked him not to order, “when you meet the really hardcore fans, you do think, ‘What are you on about?’ Basically, they’ve misunderstood us. They’ve decided we’re certain types of people, and we’re not. It’s absolute nonsense.” Previous releases such as the so-called ( ) album, which had eight tracks but no song titles, helped to cement a perception of a band who were aloof, glacial, earnest and cerebral.

“Oh, I know,” Birgisson sighs. “But there’s not one thing that’s intellectual about it.” Writing, recording and performing their music is, he says, “better than sex or anything - it’s obsessive, it’s euphoric”. So, he’s never found himself analysing his songs, dissecting them? “No, not once,” he chides. “Of course not. You sit with your guitar and just play - no thinking at all.”

Why does he think the misconception has occurred? “I think the press may be too powerful,” he replies. (More than anyone I have ever interviewed, Birgisson becomes literally rigid with caution when you switch on a tape machine in his presence.) “In Britain, the press is really weird - the weirdest in the world.”

Switch the machine off, though, and the singer is a clown, flirty, bibulous and gossipy. The evening before, a rough, nighttime canal trip on an open-top tourist boat had been eased by heroic quantities of fierce vodka. Sigur Ros are many things, but aloof – the on-record Birgisson excepted - is not one of them.

Holm, downing white wine with the caviar, thinks that Iceland, so celebrated as a hot house of creative talent, has become complacent,and seems content to put a few nosesout of joint by saying so. “Anyone you meet in the street is doing something creative,” he says. “They’re never just doing their nine-to-five job. In my opinion, it’s sometimes a little bit fake - you know, people just doing it because everyone else is.” Was that why they made their new album abroad: to escape? “Oh,” Holm concludes drily, “I think we were running away from ourselves more than from Iceland.”

The new album’s cover art - a photograph of several naked people crossing a motorway and scampering towards distant hills - was another sign that things had become a little lighter, in terms of how Sigur Ros projected themselves. It also captures the impulse to strip off and run back to nature, most forcefully summoned by songs such as Festival and Within Me a Lunatic Sings. The latter contains a pause - when, for less than a second, sonic action is suspended before bursting back into life - that may be the most exciting moment in pop music this year.

America was aghast at the buttocks. “They put a sticker on it,” Holm says, shaking his head, “which just covered the rear parts of the people in the photograph. I mean: stupid?” The video for Gobble-digook, which also contained its fair share of frolicking and nudity, was banned by YouTube. “Which is bizarre,” Holm continues, “because you can probably see people getting hit by cars, or beating up old women in the street, but a few naked people running around isn’t allowed.”

Sigur Ros have come down, as the agnostics would no doubt have it, from their lofty glacier and made an album teeming with joy, fun, spontaneity and three-minute songs. And what happens? The Americans censor them, the Russians give them fines and the internet bans them. “It makes it more interesting if it’s not easy,” Birgisson says. “Now, where’s my posh playgirl?”

Within Me a Lunatic Sings is released tomorrow on EMI

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