Joanne Robertson seems shy of the mic, but the brilliance of her voice – fragile, a little gritty, but capable of flight – makes you wonder what she's afraid of. Propping her lyric book up on a makeshift lectern, she stands a little awkwardly centre-stage and you could draw a line straight from the top of her constantly-bowed head to the peak of the ramshackle Yard Theatre's corrugated ceiling as she spins stories on her guitar. She barely shifts from the spot through the opening set, which buries the saddest sounds in the middle. If the twenty minutes feel a little like a rehearsal, what with between-song tunings and stops for hand rubbing (it's very cold in here), it's one I'm glad we were invited to sit in on.

Idly wondering how long it'll take for this wonderful venue (former industrial space now imbued with valuable artistic cred, perched precariously on the edge of Olympoville) to become swanky pads for pros, I hardly notice when Mirrorring's charming set-up is brought on-stage. It includes a hot water bottle and two candles, plus a mug of something warm. Each also has a vital battery of effects pedals, the source of much of tonight's magic. When the thick fog of 'Fell Sound' finally descends, all sense of being in a venue with other people disappears, and both Fortino and Harris, lit only by flickering flame, begin work.

On the heart-rending, Fortino-led 'Silent From Above', each of the 'one of these days' lines grows a little more pleading than the last, but Fortino barely moves, and Harris whispers, two inches from the mic, to back her up in its thrilling final moments. It's the only individual performance tonight that gets a round of applause, so distinct and straightforward is it in comparison to the rest of Foreign Body, which gets an airing in order, in full, without gaps. Each song is left to bleed into the next, eventually producing a rather numbing effect. 'Cliffs', already nudging ten minutes on record, swells into a giant, its distinctive Fortino guitar-line eventually left to the mercy of pedal-manipulation. A sound like raindrops on glass appears at one point, clicking and cracking on top of pulsing, rhythmic drones. A chime like wedding bells lingers, and as the track is stretched ever-further outwards, a couple of audience members shuffle off, perplexed.

I only ever play Foreign Body before sleep – it doesn't seem to work while being repeatedly elbowed by a Solutions Architect in a Barbour jacket on the Central Line – so it's a strange experience taking it out of that setting, sharing it with other people, and seeing what contribution each artist makes in real time. Largely, they're inseparable, although more often, Harris strums, while Fortino picks. Each appears to be in her own world, but their contributions coincide perfectly.

The entire set, like Foreign Body itself, and like each artist's own work, is really a lesson in hushed restraint. Fortino's towering high vocal notes on 'Waves' could so easily be a scene of gut-wrenching drama, but the charm is in the distance. Putting down the guitar for the 'Mirror Of Our Sleeping', a wave that never quite hits you, Harris takes to the Wurlitzer, a simple melody set to the fizz of Fortino's TV static. Here, her vocals are as close to a breath as you can imagine, and there's no climatic ending, just a gradual climb-down. It's an unsettling way to end; only Harris returning to the stage to blow out the candles lets us know they've finished.