Bright lights hover above the on-stage pair like fiery suns drawn close by a gravitational pull beyond the wildest comprehension – scalding, burning, hot, yet Dan Auercach keeps his arms inside the sleeves of a denim jacket as he wriggles immediate, yet so archaic their patents ran out long ago, riffs from his selected six-string. His sweat evaporates away as soon as it’s shed, such is the heat from on high. The duo – drummer Patrick Carney sits to his partner’s right, his left as we see him, hammering away as if battling Goliath himself – stand bathed in ever-changing multi-coloured illuminations. They look like two tourists lost on Blackpool promenade, wandering streets aimlessly, dazzled into instability and disorientation.

Yet there’s no bracing wind blowing in from the Irish Sea, no smell of seafood and chips drenched in cheap vinegar: The Black Keys have arrived at the final stop of their UK tour – a promotional jaunt to sell the songs of their latest collection Magic Potion to a surprisingly packed-to-bursting house – and discovered banks of Londoners ready to receive. This crowd is ravenous for the band’s primal, blues-fuelled and booze-soaked rock; they’re standing on tip-toes and tumbling over neighbours’ beers to get a glimpse of the objects of their present attentions, the Akron-spawned writers of four-minute fuzzy-punk pieces that defy year-specific categorisation. Where are we, exactly? 1969? 1973? 1998? Could be any of the former and any time you choose to insert as an unwritten latter: despite their sole shortcoming of note, The Black Keys’ churning and grinding retro-rock is pure and true. It’ll be as credible tomorrow as it was thirty years ago.

But the duo’s swaggering good-time set does feature a significant fault, that as-yet-undescribed shortcoming: they play for too long. Even when fan-favourites appear either side of cuts from the aforementioned long-player of this year, a sense of repetition and regurgitation bubbles up from lager-full bellies come the forty-minute mark. Also, the venue, although full, feels too expansive for The Black Keys’ one-dimensionality – they’ve not the power, the manpower, to successfully fill a room of this size. Those flaming balls of electric heat and light need to be at least a few feet lower for this to feel like an interactive experience; as it is, Carney and Auercach’s zero-score for stage presence does them no favours when said framing of their own slight frames is so massive. They’re twin Jameses stuck within a peach beyond gigantic proportions.

The clock ticks past the we’ve-been-here-an-hour mark: déjà vu after déjà vu leaves the non-hardcore aficionados – the recently acquired that had heads turned by Magic Potion but little preceding it – wondering what the rabid few’s fuss is about, exactly. Yes, The Black Keys are very good at delivering flaming guitar grooves and rollicking drum workouts, and match that to some fantastically roughed-up vocals; it is often impossible not to nod along furiously. But such is their habit to paddle in ever-so-shallow waters, compositionally, that it all grows a little substance-less in no time at all. The Black Keys are great fun for a while, but their shtick begins to sound sour rather sooner than later.

It’s ironic, maybe, that the duo would sound at least two-dimensional if fleshed out to a trio. As tautly accomplised as they are, The Black Keys are never likely to expand the boundaries they set out their particular stall within without also expanding numerically. There is a fear, one stirred by tonight’s set, that the pair’s music may stagnate entirely before trickling into the mainstream, where it almost certainly belongs.

Under these incessantly oppressive lights, it’s a wonder it hasn’t turned bad already.

(Thanks to a delay-riddled Tube journey, DiS only saw two songs by Blood Meridian, hardly enough to base a review on. Both songs were good, mind.)