Purity Ring, St John's at Hackney, London

Few highlights struggle through a soupy, samey, set in Hackney

It's the sort of gig where every appreciative tweet comes with an instagram snap attached. And there's no denying that duo Purity Ring look the part in St John's Church in hip Hackney, lit by flickering candlelight and cocoon-like lamps suspended on branches over the stage, which pulsate with multicoloured lights. Much of the sound is beaten out on an impressive drum pad-come-lighting display, with eight hittable wasp-nest like domes, which glow as they're struck.

Purity Ring are Megan James, who, swirling about with long reddish hair and longer black robes, looks like Florence Welch's cooler, gothier sister, and Corin Roddick, who wields drum sticks with a flourish. Despite the name, and venue, Christian rock this definitely isn't – who knows what Jesus (or indeed the Sunday congregation) would make of the conspicuous consumption their heavy beats prompt in a wasted Thursday night crowd.

Purveyors of icy-cool electronica, Purity Ring's brilliant first album, Shrines, is part delicately hewn synthesised tremor, part weird ethereal lyrics, and part menacing bass rumble. That last element is in full force here - it vibrates up through the church's floor and pews - but much of their precision is lost. While the acoustics at St John's can be lovely, Purity Ring's already wobbly reverberating sound becomes swampy.

Heavy on echo and effects anyway, the vocals are not just mysterious but completely incomprehensible; the effect is somewhat sub-Bjork, as a friend uncharitably but accurately puts it. The soupiness also has the unfortunate effect of making all their songs – frequently dragged out to fill an hour's set – sound samey, lacking snap, crackle or pop.

Three songs stand out, clear or distinct. 'Obedear', thanks to it's niggling, high, plinking melody and weirdly looped vocals, steps up, while 'Ungirthed' matches rapid-fire lyrics - “Ears ring and teeth click and ears ring and teeth click” - with fluttering delivery and crispier beats, and sees James whacking a big light-up bass drum with aplomb.

'Fineshrine' is their last and best song - and respect for finishing on a high rather than faffing about with an encore (they don't do much in the way of stage small-talk either). It's a very now sounding mixture of echoing percussive bursts, swelling synths, catchy melody and lyrics of creepy intimacy: “Get a little closer, let fold/Cut open my sternum and pull/My little ribs around you,” sings James by way of chilly enticement. It's about the only moment when they do truly envelop us.