I guess it’s become a cliché over the years that, when a Bristol band affiliated to trip-hop make a comeback, they should be somehow darker, and heavier, as if the magisterial doom that they all conjured up from the start somehow wasn’t enough.

It can be a pretty boring strategy, to be honest – as anyone who, like me, witnessed Massive Attack trying to play live the hamfisted paranoia of “10,000 Windows”, or watched Tricky stumbling round the stage of Hackney Empire in entire darkness at some point in the late ‘90s.

I mention this because, of course, Portishead’s long-awaited “Third” has been acclaimed, not least by me, as some kind of stark, awful masterpiece – perhaps the most creatively successful record any of these artists have made since their early ‘90s heyday, but a pretty grim listen if you’re not in the right mood. Coupled with the fact that Portishead have hardly been the most forthcoming of this already secretive clique – Beth Gibbons, famously, doesn’t do interviews, for a start – and the first London date of their comeback tour begins surprisingly.

Far from hiding in near-darkness, the six members are bathed in a harsh white light that makes the stage look like either a rehearsal room or an operating theatre, depending on your state of mind. There are screens behind them, showing the band close-up, too, though since they mostly focus on a bit of drumkit, you’re unlikely to catch a glimpse of Gibbons’ soul quite so easily.

It is, though, an interestingly artless way to present a band who have thrived on mystery, a mystery often generated by their absence rather than their active participation. The gist, I suppose, is the same as how Geoff Barrow and Adrian Utley present themselves in interviews: we’re musicians, this is what we do and you can read into it what you like, we’re just getting on – slowly, fastidiously – with our job.

So anyway, here’s Portishead, and they’re playing “Silence”, the immense opening track of “Third”. Barrow has some kind of synthdrum kit next to his decks, and is locked into an urgent motorik thump with the drummer, a mighty double propulsion that reminds me a little of Tortoise. Gibbons has her back to the crowd, swaying. After a while, the beats pause, and she starts singing with a precise, exquisite agony. It’s superb, if nowhere near loud enough.

As the show progresses, I find myself in the unusual situation of wanting to hear the new songs while the audience – though hardly the prim dinner party set habitually stereotyped as Portishead fans – are understandably pleased when those mournful old favourites are wheeled out. There’s a strange mass singalong of “Nobody loves me” during a note-perfect “Sour Times”.

But the most curious thing about hearing songs like this, and “Numb” and “Glory Box” (where Beth Gibbons’ crotchety, Holiday-ish vocal on the verse seems incredibly mannered these days), after such a long time is how they have a patina of authentic nostalgia now, as well as all those affectations of dust and crackle. They have, basically, aged well. Towards the end of “Glory Box”, the song falls down into a black hole of psychedelic dub, and I’m left wondering, embarrassed: was this always there? I need to play some old Portishead records this weekend.

Of the old songs, however, a clanging “Cowboys” is the most satisfying, because it’s closest in tone and spirit to the new stuff. If something like “Over” has an atmosphere of approaching menace, then on the likes of “Machine Gun” and “Threads”, the menace has arrived, and it’s exhilarating. “Machine Gun” is exceptionally brutal, with Barrow sending out martial volleys on his synthdrums, facing off against Utley on a beautiful old analogue synth. “Threads”, meanwhile, resembles a devastated rethink of the old style, with a piercing, sustained string sample gradually being overwhelmed by Utley’s doom chords until, finally, they miraculously resemble Sunn 0))).

It’s still not loud enough, mind, and technical problems with Barrow’s mystifying rig mean that there’s a ten minute hiatus early in the set, a neat reminder that you have to wait a while for Portishead songs. When they return, “The Rip” starts with a beautiful pastoral passage reminiscent of Gibbons’ album with Rustin Man, before driving away into a gorgeous motorik passage, Barrow putting down his guitar for another terrific passage of dual drums.

By the end, “We Carry On” has ramped up the intensity even further, the Silver Apples synth being assailed by Utley’s slashing guitar. The restless Gibbons, who engagingly never seems to know what to do with herself when she’s not singing, has given up stalking the stage and is now down with the audience, admiring her band. Not a slick, untouchable bunch of musos, as erroneous reputation might suggest, but a human and fallible group who have spent a decade absorbing some heroically extreme noise, worked out a way of incorporating it into their own sound, and come up with some of the best music of 2008 thus far. Not all gloom, then, clearly.