Over this weekend the spaces of London's Royal Opera House will be transformed by strange sounds, vaguely operatic, vaguely foresty, thoroughly chilled. The ambient atmospheres will be made by Scanner, who calls himself a “cultural engineer” and has made sounds for morgues, dances, Philips wake-up lights and chill-out rooms in clubs, during an extraordinarily eclectic career that seems to exist somewhere on the very edge of technology.

If there's one festival in Britain where people are ready for the rain, it's the Green Man. After all, nobody goes to the Brecon Beacons to sunbathe, right? The weekend, which began the spate of boutique and specialist festivals that dominate the summer season now, remains one of the most spirited in the UK, and its crowd seems to be one of the hardiest even when, as this year, the deluge is near-continuous.

It’s been a while since I’ve seen an audience go quite as bonkers as this one.

Steven Ellison is one of the most fascinating figures in modern music. Son of Motown songwriter Marylin McLeod and nephew to Alice Coltrane, he's inspired in equal part by his own musical heritage, the slow-and-low hip hop of his home state of California, and British electronica and drum and bass. His fans include Damon Albarn, Erykah Badu and Thom Yorke (the latter appearing on this year's triumphant Cosmogramma album), his Brainfeeder and Low End Theory collective of musicians and DJs are among the hippest on earth, and the world is pretty much his oyster.

The Green Man festival takes place this coming weekend at the Glanusk estate near Abergavenny in the rolling hills of the Brecon Beacons. What begun in 2003 as a glorified gig for the husband and wife duo It's Jo And Danny has become the very epitome of the 21st-century “boutique festival” - indeed is very possibly responsible for that concept itself.

I don't know exactly what they do in the music classes at Putney’s Elliott School, but it seems to do the trick. Fleetwood Mac's Peter Green went there 50 years ago and now, after admittedly a bit of a lull, the school is positively spitting stars out by the vanload. Kieran Hebden, aka Four Tet, attended, Hot Chip's members are Elliott alumni and The xx are the latest schoolkids on the block, with their self-titled 2009 debut album tipped to be a serious Mercury Prize contender.

Onstage last night, however, the Twilight-style black-garbed trio of vocalist/ bassist Oliver Sim, percussionist Jamie Smith and vocalist/ guitarist Romy Madley Croft revealed the band's limitations as well as their strengths as they studiously worked their way through most of their only long player. There is a fragile beauty to the songs when played at home, but in the flesh most of their material starts to sound a little bit, well, samey. At its best it is timeless minimalist pop, pared down to its bare bones, Chris Isaak meets Philip Glass, as on "Crystalised". But elsewhere there is too much of The Cure and New Order at their gothiest gloomiest. And the simplicity is not always sophisticated. "Heart Skipped a Beat", with its childlike hook, keeps threatening to turn into "Three Blind Mice".

It is never a good sign when one is making notes at a gig and one finds oneself writing, "Must call dentist tomorrow about daughter's teeth." Despite, or maybe because of, the glorious white-walled architectural setting of Somerset House, there were just too many distractions unless you really, as Howard Devoto said in a different context, wormed your way into the heart of the crowd. On the fringes there were those triple threats of the modern live concert, people taking photos of each other, people so excited by the gig they had to ignore it and text their friends to tell them how excited they were, and people nipping off to the bar. Though regarding the latter sin, in mitigation the bar was undeniably appealing, due to the venue organising a speedy and fair queuing system very similar to the one in my local post office. Not that they serve Carling on tap at my local post office.

The still relatively rare sight of a woman on lead guitar is more refreshing than any cold lager, but Madley Croft does little to add to the recorded versions of their songs. Sim is a lithe, lively bassist, bobbing and weaving around the stage as if ducking imaginary missiles, but his banter is largely limited to talking about the "funky house" before the band's positively glacial cover of R&B chanteuse Kyla's "Do You Mind". Only a few tracks really stood the test of live performance. "VCR" – a song title The Human League would surely use if they were starting out today – had a beefed up feel compared to the frankly plinky plonk version on their album and "Islands" retained its itchy, infectious vibe thanks to a nagging riff and Madley Croft’s whispered vocals cross-cutting with Sim’s drawl.

Frustratingly though, the threesome never quite built up more than a functional head of steam. At Glastonbury recently they were joined onstage by Florence Welch minus her Machine for a radical unpicking of "You’ve Got The Love". It would have been a wonderful way to finish with a flourish last night. Instead we got sparkly glitter shot out over the audience and a recorded version of "You’ve Got The Love" as everyone headed for the bus. If any Elliott School music teachers were present I hope they gave their ex-pupils good marks for effort but less plaudits for charisma.

Overleaf: watch The xx perform with Florence Welch