THE XX, THE CORAL | livemusic.fm

How apt that London's heatwave should break and that the sky should turn a glowering bruise-blue in time for the xx's valedictory open-air set at Somerset House. Every summer, this former Inland Revenue HQ doubles as a posh framing device for a varied slew of acts; this year's line-up is the most comprehensive yet. Only N-Dubz (who played here last week) could be more incongruous in this courtyard, so redolent of the entitlement of empire. The xx's music is intimate, working at the level of bodily tissue rather than public property. Live, they remain categorically unshowy – three static figures dressed in their trademark black with just two "x"s lighting up their stage set. Near the end, they fire off some confetti; most of it is black.

Even if you haven't bought the xx's eponymous debut, you will, by now, know their music well. The lustrous, nagging, near-lubricious goth'n'b of last August's xx insinuated itself into countless TV soundbeds, as well as most lists praising the best albums of 2009. The ubiquity of songs such as "Intro" or "VCR", both dispatched hauntingly tonight, is second only to Florence and the Machine's "You Got the Love" (which, of course, the xx covered). Glastonbury saw Florence join the trio on stage, but no such love-in occurs tonight. We only get the recording as exit music.

The band have been on the road for a year and half and this unspectacular but fluent gig marks the start of a gradual withdrawal from public duty to nurture their next album. "This is a moment of recognition of everything that's happened in the past year or so," notes singing bassist Oliver Sims, in an outburst of garrulousness quite alien to the xx thus far. Perhaps he is finally getting used to the idea of people liking his band. Xx – named for their age, 20, but also just one "x" short of outright carnality – has gone gold in the UK and sold upwards of 200,000 in the US. Their performance in front of 40,000 people at Coachella saw Jay-Z and Beyoncé hanging on their every sigh and R&B-indebted beat from the photographers' pit.

So there are no new songs or at least none ready to road test. The only musical development? After undersinging softly for the length of the gig, Sims and Romy Madley Croft (guitar) suddenly emphasise the word "fantasy" (on "Fantasy") until it sounds like a shout.

"I don't suppose there are any funky house fans in the audience?" quips Sims before triggering a typically yearning, understated cover of Kyla's "Do You Mind" (contrast theirs with the original, which virtually begs xx-ification). The crowd's sporadic attempts at dancing eventually ebb into a gentle sway. Everything about the xx is latent, not manifest; their electronic builds never, ever spill over into jubilant release.

Like the xx, the Coral were once the surprisingly young people making preternaturally evolved music. Nearly a decade on from their early EPs, the Coral have just released their sixth album, The Butterfly House,probably their most cogent and satisfying yet. They have lost, regained and parted company once more with guitarist Bill Ryder-Jones, leaving, oh, just the two or three guitarists to carry their masterful, country-tingedpsychedelia. Singer James Skelly (bowl cut intact) and skinny, precise lead guitarist Lee Southall change guitars after every single song. If he were paid by the piece, their guitar tech would be the best-remunerated roadie in rock.

It's worth dwelling on Southall, a precise player who executes guitar solos with all the mechanistic selflessness of the pre-rawk era. Psychedelic music is so often associated with derangement, but when the Coral wig out – as they do on a final "North Parade" – there is no loss of control, just an expert corralling of wild vintage sounds on a clearly staked-out path.

If we can just assume that verve, a sense of mystery and sonorous thrum are standard on Coral compositions as far apart as "Dreaming of You" (from their 2002 debut album) and "She's Coming Around" (new), then two fragile Butterfly House beauties stand out tonight. "Falling All Around You" finds Skelly without a guitar, crooning a stripped-down, country-tinged ballad, Nick Power's organ chiming gently along. "Walking in the Winter" is even prettier.

Every time the Coral come around, you wonder why this splendid band aren't more widely feted. In the past, it has been due to their zanier acts of self-sabotage (the Nightfreak and the Sons of Becker album springs to mind).

But now they have two perfectly canonical works (Butterfly and its predecessor, Roots & Echoes) under their belts. Thanks to Fleet Foxes, west coast music with close male harmonies is all the rage. The Coral's second coming should be around now, but you can't help but wonder how much bigger this purposely arcane and nostalgic outfit will get.