On the other side of the door is a sprawling south London estate gathered round an old factory turned business park, as oppressive an environment as you'll find anywhere in the city. To enter the airy, top-floor studio where Sigur Ros ready themselves for a European tour, though, is to be transported.
The elements of their sound are becoming familiar. A glacial falsetto voice floats across a slow cycle of descending notes, pursued by a swarm of ethereal, violin-bowed guitar and a warm wash of real strings. There's the shuffling beat, some distant, echoed radar-like blip, a feeling of light and space that fleetingly conjures artists ranging from Spiritualized and the Cocteau Twins to Bach and the insane but brilliant Gesualdo.
The last time I saw them playing this song, 'Svefn-G-Englar', the Icelandic four-piece were supporting Radiohead in Paris, commanding the wildest applause I've ever seen an unknown band get. Bassist Georg Holm remembers that night, admitting that, 'when I walked offstage, my legs were shaking'. Before long, Rolling Stone would be naming his group's second album as one of last year's 10 best, despite the fact that it is still only available on import in the US.
Still all 25 or under, Sigur Ros have been together for seven years, variously working in child-care centres, TV studios, steel works and homes for severely disabled people, or studying politics at university, to survive along the way. They've spent the past few years as their country's most popular home-grown band, thanks in particular to that second album, Agaetis Byrjun - which, despite being sporadically inspired, still gets nowhere near capturing their live magic.
The alien quality of their sound owes more than a little to vocalist Jonsi Birgisson's penchant for singing in a mixture of Icelandic and a tongue of his own invention, which he likes to call 'Hopelandish'. The key to Sigur Ros's post-rock rapture is that while you can't understand what he's saying, you always feel you know what he means.
That one of the freshest bands to have emerged in a while should come from Iceland doesn't surprise us anymore, not since Björk and the Sugarcubes and various contemporaries like GusGus first established the nation's talent for vivid, non-conformist pop. Nevertheless, the present crop looks to be particularly promising, with Sigur Ros being closely followed by the brilliant Blake, Mum, Kanada and old Blur favourites, Lhooq. All are helped by an active but compact music business, whose infrastructure they control. Record company accountants and the stifling short-termism they bring hold no sway over what goes on.
Hilmar Hilmarsson, a doyen of the Reykjavik scene, cites the country's long domination by Denmark as a factor in the delight his people seem to take in creativity and invention.
Sigur Ros's articulate bass player, Georg Holm, will add that there are no surviving folk instruments. Indigenous music is vocal and you wonder whether the characteristically Icelandic willingness to make mischief with genres and stylistic conven tion owes something to this. The tiny 250,000 population is perhaps equally important. 'It's a small place, so things get jumbled up and having clearly defined boundaries is difficult,' is how he puts it. Ergo, the kind of tribal purism that always defined our culture is not possible in theirs. For instance, Birgisson, who carries the disarming expression of a naughty child caught with his hand in the biscuit tin, will tell you that his favourite vocalists are Leonard Cohen and the jazz trumpeter Chet Baker.
Holm will mention Cohen, too, explaining: 'I never considered him depressing or sad; I always thought of him as uplifting.' (This is a very Scandanavian sentiment; for one Icelandic reviewer, Sigur Ros were so beautiful in concert that she had to run to the lavatory to throw up.)
But if you inquire as to whether he listened to classical music as well, he looks at you as if you've just asked whether it ever gets chilly in Reykjavik. Eventually, he replies: 'Yeah, yeah.'
Sigur Ros play Brighton, St George's Church, 21 April; Leeds, City Varieties, 23 April; London's Sheperd's Bush Empire, 24 April