Oliver Sim casts an approving eye over the spacious home of the iTunes festival. "The last time we played here, we were in the basement," he recalls. "Things are looking up."

As understatements go, it's a big one. Since the release of the xx's eponymous debut in August last year, Putney's finest have moved from latent underground heroes to mainstream heirs apparent, winning critical plaudits worldwide and nabbing the Guardian first album award.

It's the kind of success story that keeps kids buying guitars, but the past year hasn't been all kind. The departure of original member Baria Qureshi trimmed the xx down to a trio, while their seemingly permanent tenure on the road was temporarily halted in February to allow Romy Madley Croft to mourn the loss of her dad.

Now, after a spring spent playing American festivals, and with a summer full of European ones, Sim is simply happy to be in London. "I can't tell you how nice it is to be back home," he says with a sigh.

Sentiment aside, the black-clad band strive to bring an album to life on stage that is essentially a solitary and subtle listening experience. As the brooding instrumental Intro unfolds, the audience listen quietly, mesmerised by Croft and Sim's breathy murmurs and the nimble fingers of Jamie Smith on electric drums.

Crystalised prompts a hushed singalong, while two big screens on the sides of the stage reveal both Croft's inscrutable expression and the vulnerability in her eyes. Her soft, innocent vocals waltz with Sim's hoarse, seductive moan on Islands and VCR, but physically they couldn't be further apart. He moves fluidly with his bass guitar, striking poses and shaking his hips; Croft is rigid, barely swaying her shoulders.

When the lifelong friends come together, almost knuckle to knuckle, Sim towers over Croft, but it's her spine-tingling guitar that holds each delicate song together. It's not enough, though, to prevent the rising buzz of chatter from an audience tiring of the xx's minimalist magic.

Sim's appeal for "funky house fans" is met by apathy, but a cover of Kyla's 2008 favourite Do You Mind, during which Smith bashes a real bass drum and snare, wins attention. Sim's cymbal-bashing antics during the stunning Infinity keeps the momentum going, and by the time of glitter-adorned encore Stars, the xx have ensured that while their sound and stagecraft could do with being bolder, they won't be going back to the basement anytime soon.