A sell-out, or should that be a shell-out, return to London last night threatened to be an ignominious flop for Chicago group Tortoise at the recently reopened Garage in Islington. It all unfolded in front of a heaving crowd desperate for a sighting of the group that spearheaded the birth of post-rock with that “secret” but crucial element: whisper it, jazz, hidden away in the sonic stew. After all this was the group that had its origins back in the 1980s, according to the band’s own chronology with drummer John Herndon, still looking fresh faced and very polite last night to the audience, and magisterial but serious-looking bassist Doug McCombs spending “many hours jamming on Africa Brass”.

Unfortunately last night the band was dogged by what seemed to be a faulty lead attached to Jeff Parker’s guitar which stopped the band in its tracks a couple of times early on. But in spite of this the gig turned out to be a triumph, with a freshness and improvising candour that would put many an official jazz group to shame. Parker, better known to jazz fans for his work with Chicago Underground, scythed his way impressively at times with great jagged cuts, settling down eventually after all the hassle. But when the band moved forward after its signature instrument switching and musical chairs – one minute drums, the next synths, the next vibes – and moved into textural synth territory, it provided strong narrative moments built on subtle shifts. There was even an unsentimental melancholy as if the superb rhythmic facility of John McEntire on drums (especially when joined by Herndon in a two drum cross-stage head to head) wasn’t enough.

Beacons of Ancestorship, their latest album certainly has its bright moments, and this gig was very much like the album. The technical problems almost spoiled it but this group has never been more vital as an inspiration for new Brit-jazz groups especially the likes of Troyka and the new wave of prog jazz. The music has moved on and Tortoise are a vital reminder of this. Sometimes it takes outsiders, who don’t give a damn for genre, like these Chicagoans, to shake it up.

– Stephen Graham