“What? I thought I was watching CocoRosie?!”

Nope, that was Brooklyn based minstrel Rio En Medio, and it’s a foregone conclusion that she’s on the same page as the sisters Casady - gunning for Cocteau Twins-esque eerie falsetto and even hailing from Bianca’s Voodoo EROS stable. On any other bill these harp-driven narcoleptic lullabies would probably hold in their own right, but tonight this baffling near homage makes little sense. It's all chicken and egg stuff. Luckily, before long, an unlikely saviour comes to The Arches' aid…

“It’s Jones from Police Academy!”

With his work cut out in keeping this fairly incestuous affair moving, French beat-boxer Tez rises to the challenge with nothing more than his larynx to keep him afloat; aping the likes of Dead Prez’s ‘Hip-hop’, Roots Manuva’s ‘Witness (One Hope)’ and Prince’s ‘Kiss’, he masterfully emulates the bass components from all and literally makes the walls shake with the thunder harnessed by his amplified throat. Sure enough, Tez swiftly succeeds in single-handedly lifting the mood and although Biz Markie would have had trouble on his hands were he here, this moment of triumph proves short lived as something of an attention deficit breaks out en masse, just in time for the main event…

“You guys get the prize for most talking during a show, ever,” frowns Bianca Casady.

Worlds away from the palm-treed backdrop where these eyeballs last saw CocoRosie jamming with their mum at Coachella, the darkened hollow offered by this abandoned railway tunnel makes for perhaps none finer a venue to take in their kooky yet considered melodramatic capers. Unhindered by the sound problems that previously dogged them in the desert, here they face an unlikely new challenge: the fickle fuckwittery of about half of their audience. Some wave around flashing cameras for the entire gig, others shout to each other in Spanish down the front; in any event it bounces off the walls and visibly frustrates the remainder who care about where their tenner went.

Despite the ruckus, Tez is back; on hand to provide sporadic percussion and put his capabilities fully to the test in order to provide any instrumentally unattainable effect that the sisters might need. But this isn’t simply about the vocal acrobatics: “You want to fuck me, I already know,” purr the sisters in unison, placing the embarrassingly misogynistic shoe of Snoop Dogg’s libidinous words firmly on the other foot. And, suddenly, the chatter eases up for all of a minute.

“Calling all kids, grown-ups are crazy. Nana-nana-na-na, your mama’s on crack rock!”

Though the delivery is resolutely playful - Bianca's occasionally grating Björk-isms colliding with Sierra's classical side - the duo remain hell-bent on testing the limits of what moral reason and religious dogmatism will allow. Slight re-workings of songs like ‘Jesus Loves Me’ and ‘Japan’ pretty much perfectly encapsulate the way in which CocoRosie set out to bewitch as much as they're in the business of bewildering this coffee morning gone mental.

Three albums in and they're still an enigma. With nothing much finite about them, the allure of their neurotic blend of folk-hop remains simple: it's easy to get caught up in a good mystery.

Photograph by John Lewis