It has been 10 years since the world last heard from Portishead, when TV producers 'turned our sounds into a fondue set'. A stunning new album could even herald the rebirth of the Bristol scene. An exclusive interview by Ben Thompson

This is how the new Portishead album starts. A friendly voice says something vaguely introductory in Brazilian-Portuguese. There's a bit of subdued chatter in the background, and the reassuring plink of a distant piano, as if you're arriving at a half-empty Latin nightclub. Then a huge pummelling beat comes in (Geoff Barrow insists that he was 'massively unhappy' with this rhythm for many long months, but it sounds pretty unstoppable now). Sawing strings summon up a demonic echoing cowbell, before this in turn gives way to ominous slashes of spaghetti western guitar - the sort of thing you'd expect to hear just before a hired gun played by Lee Van Cleef accidentally shoots an innocent child.

Two minutes and 10 seconds in, the scene is finally set for Beth Gibbons's vocal to make its entrance. But however effectively the listener has been softened up for this momentous event, no one will quite be prepared for the pitch of ecstatic anguish at which her voice announces itself. 'Wounded and afraid inside my head,' Beth flails poignantly, as a Tardis seems to take off in the background, 'falling through changes ... Did you know what I lost? Do you know what I wanted?'

It's stunning stuff. And this is just the opening number. Later on, once Third (for that is the title: it is, after all, Portishead's third album - well, if you don't count the live one) is properly up and running, it features a run of five or six songs which are not just worthy of the records this band were making 10 or even 14 years ago, but feel like the sonic destination which they were always meant to arrive at.

Being amazed by the music of Portishead was not an activity at the top of many people's agendas for 2008. A couple of years ago, when this slow-moving West Country ensemble tentatively emerged from retirement by contributing a track to a poorly conceived English-language Serge Gainsbourg tribute LP, they sounded like a band on the verge of total creative exhaustion. When checking in with Barrow's blog in the run-up to December's comeback as curators of the more-underground-than-thou ATP festival at Minehead Butlins, the omens were no more propitious. 'This album has been like watching Lost,' he pronounced gloomily, 'a never-ending journey with few answers.'

'Wish we had more time,' he yearns poignantly at another juncture. And why shouldn't he? It's only been 10 years since their last record, after all. What you might call Bacardi advert time-frames have always been an issue with the music made in this part of the world, however vehemently those responsible might protest otherwise. And with Tricky and Massive Attack also poised to release new albums, perennial late starters Portishead find themselves in the unaccustomed position of being first out of the blocks.

Far from being unnerved by his band's decade-long shore leave, genially rumpled bed-head poster-boy Barrow seems to view it in an entirely positive light. 'Over that 10-year period,' he explains, 'the pressure to have successful records obviously dies. Which is really good, as then you can just do what you want.'

The home studio on the top floor of Portishead guitarist Adrian Utley's five-storey Georgian house in the classy district of Kingsdown supplies an abundance of circumstantial evidence as to how that decade might have been spent.

A lovely old harmonium Utley bought off eBay for £29 squats elegantly in one corner. In another, there's an ancient synthesiser signed by its designer, recently deceased musical pioneer Bob Moog. Anyone who has even a passing familiarity with Portishead's bespoke blend of antique futurism and ambient savagery would expect them to inhabit a world full of lovely-looking musical instruments. But there's an intensity to the atmosphere in this room which suggests it's home to a band who care about substance more than style.

'We've had some dark moments in here,' admits the usually ebullient Utley. 'Discussing things for hours to try and make the world around the music completely solid, then listening back to stuff and not liking it, and then not listening to it and talking for hours about biscuits.'

The vista that unfolds across the long picture window above the giant wooden mixing-desk - a breathtaking view down the slope to Bristol's notoriously unsatisfactory city centre (of which more later) and beyond to the green hills in the distance - would grab the attention of even the most sceptical observer. Whatever the precise explanation for Portishead's unlikely creative renaissance, it seems probable that the complex relationship between this panoramic backdrop and the essential privacy of the band's notoriously complicated recording process is going to be somewhere near the heart of it.

When a lot of exciting music emerges from one place at roughly the same time, it doesn't usually take long for the things people say in the hope of capturing this excitement to start looking foolish. Whether it's Newport being the new Seattle, or Sunderland being the old Sheffield, the onward march of rock'n'roll history soon tends to tramp down such proudly regionalist boasts into a kind of hyperbolic mulch.

In the case of the 'Bristol sound' of the early-to-mid-1990s, however, the idea that something special happened at that particular time and in that particular place has grown more persuasive rather than less with the passing years. This is partly because the music has lasted so well - listening to Massive Attack's Blue Lines, Portishead's Dummy and Tricky's Maxinquaye a decade and a bit later, it's impossible to deny that these are three of the most resonant and perfectly-formed debut albums to emerge in over 50 years of British pop.

The particular circumstances of growing up in that city in the Eighties - from the endless social ramifications of Bristol's historic connections with slavery, to the fallout from an especially euphoric punky-reggae crossover, the enduring influence of art-school polemicists the Pop Group, and the shared hip-hop heritage of the Dug Out Club and the Wild Bunch sound-system - helped shape a uniquely cosmopolitan culture which the rest of Britain recognised but could not hope to emulate. And at a time when the kind of sophistication a broadband connection or a nationwide Italian-style coffee franchise brings are equally accessible to everyone, the idea of such a distinct mindset now seems almost impossibly mysterious.

Yet speaking to all the major players in this story as it was unfolding, it soon became clear that what divided them was every bit as important as what they had in common. 'Everyone thinks we're all sleeping together,' Blue Lines's puckish featured rapper Tricky Kid murmured to me in 1994, before bemoaning the fact that while he was working on his first solo endeavour (a contribution, as it happened, to a benefit project for research into sickle-cell anaemia), all the full-time members of Massive Attack 'came back from the pub to take the piss'. And while acknowledging that his band represented 'the first generation of immigrants that grew up in England', in conversation Massive stalwart Daddy G was quick to emphasise that 'you can't say that living here has affected us all in the same way, because it hasn't'.

Perhaps inevitably, two of the clearest and most commercially successful paths through the Bristol scene's forest of inter-personal connections were trodden by white kids. Nellee Hooper jumped ship from the Wild Bunch sound-system to become first one half of Soul II Soul, and then (for a while at least: no one seems to know what he's up to at the moment) the world's most sought after record-producer. And Barrow - teenage tea-maker and tape operator at the Coach House studios where Massive Attack recorded Blue Lines - went on first to co-produce Tricky's solo debut, and then to start his own group, named (in classic hip hop-style) after the town he'd grown up in.

The fact that this place - Portishead - was neither an LA ghetto nor a suburb of New York but a no-horse satellite town of Bristol, clinging on to the edge of the Severn Estuary by its scabby fingernails, was a reality which Barrow has scrupulously refused to overlook from that day to this. In his quest to make music which was informed by the American hip hop he loved, but also true to the place it came from, he enlisted two somewhat unlikely allies. One, Beth Gibbons, was a singer-songwriter almost a decade his senior. The other, Adrian Utley, was a veteran jazz guitarist who was rumoured to have once played with Art Blakey.

Because it's simpler, people have tended to see the subsequent development of Portishead's music as a straightforward tug of war between rough-edged cut-and-paste merchant Barrow, and Gibbons the enigmatic chanteuse. In fact, the band's creative power-base has always been more of a triangle; the atmospheric sound-scapes don't really make sense without a proper understanding of Utley's contribution.

The great thing about this apparently ill-matched trio was that each person brought something to the table that the others needed: Gibbons's striking vocal and visual presence and old-school song-writing talent, Barrow's fan-boy grasp of the mechanics of hip hop, and Utley's musicianly chops.

'Because we came from such different worlds,' Utley remembers, 'what really got me and Geoff talking to each other first was Public Enemy. As I'd come from a traditional playing-with-other-people type of background, I didn't know how these records were being made. I'd never had a sampler - they were really expensive at the time - so I just didn't understand where these amazing sounds were coming from.'

'With samples, in those days,' he continues - 'Before time-stretching and all this Pro Tools tomfoolery,' qualifies Barrow, ever the home-schooled rap technician - 'you were actually forcing notes against notes, so there was a proper clash and everything was slightly out of tune. Because you want this riff to go over that beat and you've just got to make it happen, you end up with this kind of roughness, which is what made Public Enemy or Eric B so exciting.'

'For me,' adds the older man, 'finding hip hop was a huge life-changing experience - like having a baby or something'. Even as he says this, Utley's 14-month-old daughter is taking some of her first steps across the wooden floorboards of the downstairs kitchen. And just as this road-hardened jazz-warrior was first discovering hip-hop's 'whole world of fantasticness', he toddled into Barrow coming back the other way.

Years of immersion in the crate-digging one-upmanship of DJ culture were making the younger man increasingly uncomfortable with hip hop's 'trainspotting tendencies'. And what better way to transcend your disenchantment with 'people getting excited about the first two bars of some beat that sounds like it was played at Butlins' than by forging a new creative partnership with someone who might have helped keep that beat? When Barrow was holidaying at Torbay Pontins as a child in the very early Eighties ('We never went to Butlins - it was too posh'), Utley was actually playing in the band.

Portishead were painfully aware of the embarrassment that can ensue when white musicians try to appropriate black musical innovations wholesale - 'We didn't want to be Cliff Richard singing over a loop of "Funky Drummer",' Barrow almost snarls (Bristol's biggest venue, the Colston Hall, might still be named after a slave-trader, but that's all the more reason for the city's artists to eschew a colonial mentality). So they set themselves the challenge of incorporating a sense of that distance from their source material into the very fabric of their music. 'It can't just be like a live tape played by a load of musicians,' Utley explains, 'it has to feel like it's already a record of something, like it's come from a world.'

The band's determination to be true to this underlying principle has necessitated a uniquely painstaking process of artistic evolution. Each time they've proved a particular way of doing things can work for them, they seem to feel compelled to tear up that rulebook. Thus, when making Dummy, Barrow would record a combination of samples and live music, press them on to vinyl, then scratch-mix the results to make backing tapes for Gibbons to add lyrics and a tune to. But for 1997's Portishead this methodology was no longer deemed tortuous enough, so the band invested endless months in creating their own samples from, as it were, scratch. ('We couldn't just put the needle on a record and take someone else's music,' Barrow grins painedly, 'and besides, it was more fun to do it ourselves.')

At the time, Utley described this process as being 'like trying to walk with one Wellington boot full of concrete', but Portishead's endless labours received their just reward in the end. Not commercially (the second album sold less than the first), but in the loftier realm of kudos. When 'Over' was sampled by both Timbaland and Wu-Tang Clan's RZA, Barrow had achieved his ultimate objective of giving something back: supplying actual fuel for hip hop's hungry furnace.

Small wonder that Portishead - like their increasingly fractured and discontented peers Massive Attack and Tricky - were running out of steam by the end of the Nineties. Geoff Barrow looks worn-out even having to think about this stage of his band's history. 'After we got back and mixed the live album [1998's impeccably grandiose Roseland NYC Live - probably, strangely, the best place to start for people unfamiliar with their previous oeuvre] we didn't really get together for six years ... It wasn't that we weren't speaking. But Beth got ill and moved back to Devon for a while. I got divorced, and we all worked on a few solo projects.'

The smart money, at this stage, would not have been on Portishead's most sonically adventurous and exhilarating work still being ahead of them. And when they finally got back together, things - to put it mildly - took a while to get going. 'I bought hundreds and hundreds of records,' Barrow remembers, glumly, 'sampled them, looped them up and made backing tracks ... and it just put me into a massive state of depression, basically.'

Why?

'It just seemed so backward, and like something we'd done too many times. The songs sounded OK as instrumental hip hop, but as soon as Beth started singing, it was like "Oh man, no way". The idea of us just trying to be Gang Starr with Beth on top just was not really interesting to any of us any more - her included. We ended up going back to early hip hop drum machines, because they were the only things we could really stand listening to. The idea of classic breaks that had been chopped up was not really palatable any more.'

The roots of Barrow's allergic reaction to the sounds he once loved probably lay in the unasked-for ubiquity of his band's debut album. At some point around the time Dummy won the 1995 Mercury Prize, Portishead found that the music they had lovingly fashioned from scraps of Lalo Schifrin's old film scores had suddenly (when featured in the background on aspirational twenty-something TV drama This Life) become the soundtrack to a mid-Nineties media lifestyle fantasy.

'They turned our songs into a fondue set,' he observes, disgustedly, more than a decade on. The same anger was on public display in his recent entertaining blog-spat with Mark Ronson, in which he accused the well-connected New Yorker of making 'shit funky supermarket muzak', eliciting a pithy but intriguingly off-target response about Portishead's music no longer being 'popular enough to be played in supermarkets' (an eventuality which actually seems to be a great source of relief to the whole band).

Barrow and Utley seem to agree that one of Third's subliminal themes is the 'ridiculousness' of contemporary existence. 'You've got the surface world - the absolute unreal world that everyone is supposed to live in - and then there are the actual real things that are happening, and then there's this ginormous layer of media which divides the two,' Barrow fulminates sheepishly, as if conscious that he has expressed this dilemma more elegantly in musical form. It's a neat demonstration of the contrasting personalities which give the music of Portishead its light and shade that when the topic of This Life comes up while he and Utley are chatting to photographer Harry Borden at the OMM photo shoot, Beth Gibbons pipes up from the other corner of the room: 'Oh, I always quite liked that show!'

Under normal circumstances, this would be the only quote from the notoriously reticent vocalist that would be available to us. Happily, in the course of my previous professional encounter with Portishead, in late 1994, a sudden indisposition on the part of Barrow (who'd been hospitalised with a suspected ulcer) caused Gibbons to lift her lifetime ban on talking to British journalists for the only time. Far from the Garboesque recluse of legend, she turned out to be a genial, no-nonsense character, wearing glasses and speaking with a light Devonian burr rather than a tortured croak. After rooting through a cupboard in her small terraced house in the Easton district of Bristol, she unearthed a series of unreleased Portishead songs and played them back with a self-deprecating running commentary - 'This is where I tried to rip off Sinead O'Connor' - or 'Neil Young', or 'Tom Waits' or 'a black soul singer'. Yet it seemed the harder Beth Gibbons tried to copy someone else, the more she sounded like no one but herself.

Before giving me a lift back to the station in her elegantly battered Triumph, with a pre-release copy of Maxinquaye on the stereo, Gibbons said a couple of things which enhanced her band's air of mystery far more effectively than any long-term vow of silence.

The first was that even though in some ways the loneliness which had driven her to write songs in the first place had been intensified by sending that music out into the world as a commercial product, 'if you think of something like the mannequins in Blade Runner, they only think they're human because of the pictures they hold'. The second - rather less existential, but none the less intriguing - observation concerned her relationship with Barrow.

'It may be the age gap, but he never quite knows how to take me,' she said, of the musical associate she'd first teamed up with at an enterprise allowance induction day three years earlier. 'If you asked him about me, I don't know what he'd say.'

Fourteen years later, Barrow discusses their initial meeting in similarly uncertain tones: 'She was a woman, but I was still just a boy really. I mean, the lyrics she'd written for the first song we worked on together name-checked Gandhi! I just did not have the first idea of what she was on about.'

In the intervening years, the distance between Portishead's ruling triumvirate has greatly decreased. 'We're closer now than we've ever been,' insists Utley, 'and the lines between what each of us contributes have totally blurred.' The way the band works has basically turned inside out. And you can feel the ensuing emotional thaw in the music on Third. In place of the old demarcations, there's now the sense of a rush towards a shared objective.

Some of this extra momentum comes from the beats. Take the percussive thrust of the album's opening track, for example. 'The vibe I really wanted,' Barrow explains, 'was when Muhammad Ali fought George Foreman at the Rumble in the Jungle and James Brown played. I wanted it to feel like it had been recorded somewhere really fucking remote.' But there's a further source of additional energy at work, one which (for Geoff at least) comes close to matching the intensity of his earlier connection with the hip hop mother lode.

'If you look back at the ATP line-up,' Geoff enthuses, 'it's basically a list of the music that makes our album.' Given that this three-day event traditionally offers a bill of fare so ascetic that even hardened readers of the Wire magazine are inclined to want to listen to Diana Ross and the Supremes in the car on the way home, readers could be forgiven for feeling somewhat unnerved by the prospect of Portishead's new doom-metal direction. But listening to Third, the sheer savagery of the album's numerous sonic switchbacks seems to have also shaken free the band's melodic sense as well.

The first three songs are something like you'd expect from a new Portishead album. They're kind of the same, but different - like arriving at the airport and having to carry your toothpaste through passport control in a plastic bag. But then the record really takes off, and suddenly it's taking you somewhere you've never been before.

A tune called 'The Rip' starts out like an old English folk remnant but ends up in the enchanted realm of early Kraftwerk. Another amazing song, 'We Carry On', joins the dots between the ramshackle urgency of Sixties punk and the terrifying precision of Joy Division. And let's not forget 'Deep Water' (which Utley hates): it's the most touching Steve Martin-inspired close-harmony ukulele ballad in Portishead's entire repertoire.

Back up in Portishead's rooftop eyrie, Adrian Utley points out places of local interest through the window: 'Nellee Hooper came from Barton Hill, over there, which is not a fairytale place to live. Tricky was from Knowle, which is pretty grim as well, and Mushroom lived in Fishponds.' At this point, talk turns to Bristol's endlessly postponed city-centre redevelopment.

There's a school of thought which believes that Tony Wilson's Factory Records dreamt up the Manchester of the 21st century. Can either Barrow or Utley imagine a metropolitan hub inspired by Dummy or Maxinquaye

'I don't know what that would look like,' the former grins, 'but it would be fairly frightening, I reckon.' At this point, a courier arrives with a freshly remastered copy of Third, and Barrow sticks it into the CD player and strains his ears to assess the impact of another set of infinitesimal adjustments. If Massive Attack's and Tricky's forthcoming albums sound anything like as good as this one, well, Bristol just might be the new Bristol.

· Third (Island) is out on 14 April; Portishead also tour that month.