It's one of those nights of seeing an act in the ascendant - an act whose song 'Still', now 18 months old, was the final 'Hottest Record' on Zane Lowe's Radio 1 show. Meaning that it still hasn't been superseded in hotness. The Clean EP from which it sprang showed that there's more than one mood on The Japanese House's moodboard. It's surely a good sign when you mistake the support for the main act, though - and Candy Says took this rare honour, with their live vocal processors and fashionable leisurewear. There's a classicism to their songwriting that's now draped in well-fitting electronics, with flavours of everything from Goldfrapp to Pavement enlivening Juju Sophie's indie recipes. They have their own whole little world at candysays.it and I loved their wonderfully awkward stage bants.

One of the glories of being on time for the support is when, due to my own cluelessness, someone who appears to be a roadie suddenly dons an electric and turns out to be Tamu Massif. And turns out to be indescribable. At first I'm unsure whether Dave Dixon is singing baritone or rapping, his Sprechstimme/indie-jazz approach soon reveals itself. But it does so alongside skittering drum patterns, sampled dialogue, and constant visuals rolling by on a screen behind his white-teed frame. Very interesting work – I'd download 'St. Isidore' whose languid major-seventh chords and ping-pong percussion show off his arrangements whilst not sounding like it's trying too hard.

Then the venue plays Talking Heads' Remain in Light in its entirety. Which is a pretty hard act to follow.

Let's be clear, our headliners have no members of Japanese heritage and house music ain't on the menu – Buckinghamshire's Amber Bain is The Japanese House, with other musicians held in her orbit. She has a Bon Iver-like approach to guitar: fingerpicking from which pedals sprays out digital shards. She does with this instrument things I wish I could, but I guess her USP/Bat-Signal is a series of vocal processors redolent of Imogen Heap or Mr Iver himself. Which, when combined with an overwhelming strobe light, blew me away.

She's a technical wizard, 21, and doesn't mind what you think her songs are about. Which is appropes for a band-name that encapsulates a gender-bending childhood holiday. Bain knows that the interpretation is part of the fun – but the overriding sense of a Japanese House playlist is a kind of exquisite sadness. Upstairs at the O2 is now entirely full, and her newly bolstered line-up fills the stage with unexpected swagger. There's muscular, cymbal-friendly and laser-precise drumming, warping keyboard textures, and the dual guitar knees-up in the final song is a winner. As crystalline and un-second-guessable as her arrangements are, this is a streamlined set (encore-free), all tension and reward.

With 'Clean' I can finally hear the oft-spoken-of echoes of associates The 1975 and androgenous ennui; the verse of 'Swim Against the Tide' is gentle like Joni or Damien Rice (i.e. brutal), and its arrestingly precision-engineered chorus allows her to knock sparks off that guitar. Only once does a song seem too grandstand-y, and that's 'Face Like Thunder' - even then I feel wrong as everyone in a 20-yard radius is singing along, syllable-perfect. However, until the final song Bain exchanges more words with the sound-man than the audience, and at one point this undermines the dramatic peak of a performance. 'Saw You in a Dream' might signal a change of direction for her new work – its unapologetic dream-pop has Cocteau Twins guitars, a twisty melody reminiscent of them/Spandau Ballet/the casual magic of a Paul McCartney bridge. It's only been out for a week, and hearing it unveiled live is just like viewing another facet of JH's gem-like talent. 'Cool Blue' is bright, galloping like the Fleetwood Mac Bain loves – and in the end I buy that EP, only 18 months behind the curve. It's not too late to catch up with Bain. That's if anyone can catch up.