A short drive from Bristol city centre, and just beyond a brightly graffitied rail tunnel, St Werburghs city farm feels like a scrappy little corner of liberal utopia. When I visit, it is laid out in mellow, late June sunshine. Sheep nibble grass in an enclosure beside the road, and behind them the ground slopes up through allotments and tool sheds to a pick'n'mix assortment of pink and white houses.
Portishead's Geoff Barrow sniffs the manure-heavy air outside the sheep enclosure approvingly. "It's a little haven round here," he says. "There's not many places in the city that smell like that."
All, it seems, is well in Camp Portishead. Guitarist Adrian Utley whirs up on a battered bike; Barrow (the band's multi-instrumentalist and producer) is tugged along the pavement at the end of a lead by his hyperactive terrier Alfie; and singer Beth Gibbons arrives behind the wheel of a big family car, like a mum on the school run.
The three members of a band responsible for some of the most searingly bleak music of the past 20 years radiate a feeling of domestic contentment. Only Alfie is doing any sort of justice to their reputation for emotional intensity, picking fights with passing dogs and a half-buried model of a motorbike in a nearby garden.
"Aren't you going to say hello nicely?" sighs Barrow, after another volley of barks.
But if Portishead are presenting a sunny exterior to the world, the one cloud on the horizon is that the following day the machinery that surrounds the band will begin grinding into gear again. A few days after our meeting, Portishead will step on to the stage as headliners at the Danish festival Roskilde (the first in a series of European festival dates they are booked to play), and it's a prospect, though exciting, that they are struggling to get their heads around.
"I feel, 'Hang on, I don't understand why we're doing this yet,'" says Utley later, sipping coffee in a pub garden just up the hill. "We've come from our kitchens with our kids, and suddenly we're walking on stage and playing at a festival in front of 30,000 people. How does that relate to our lives now? I'm finding it hard to come to terms with."
The band can be forgiven a certain tentativeness at returning to the live circuit. The last time Portishead played festivals in Europe was in the mid-90s; not long afterwards came a slog around America, which finished in 1998 – and almost finished them off. At the time, Portishead were close to the height of their success. The eerie, off-kilter soul music of their 1994 debut Dummy – pieced together from warped hip-hop beats, bluesy guitar textures and Gibbons's keening vocals – made them critical darlings and helped them sell more than two million records.
And yet, much like the xx's debut 15 years later, certain snippets of Dummy became irritatingly inescapable: the record's closer, "Glory Box", would frequently crop up on cult BBC2 drama This Life to reflect the existential despair of a group of twentysomething trainee lawyers living in a house and shagging each other.
But when Portishead followed it in 1997 with their eponymous second album and headed for America, they seemed, unlike many of their British peers, genuine contenders for major transatlantic success. Out of the studio and offstage, however, the wheels were coming off.
"We'd had enough," remembers Utley. "A year and a bit on the road, all our relationships had gone down the shitter. It was that whole bloody cliche, that when you're most successful, your personal life is destroying itself. It was like, 'This is a film script, a crappy story someone's written, and I'm living it.'"
No one ever formally called time on Portishead, but when the band came back from America they went their separate ways.
"It was like, 'Fuck, we got out of that one alive,' and we walked away," says Utley. "We all got our hands burnt and we didn't really want to go back in."
The strains of that period and the unwanted baggage that had become associated with Portishead were largely laid to rest three years ago when, after several years apart and several more of creative block, the band returned with "Machine Gun". A punch-to-the-stomach of a first single from their comeback record Third, its barrage of distortion buried the idea that the band produced chilled-out lifestyle music – or, in the lazy terminology of the mid-90s, trip-hop. Still, when they talk about the prospect of touring and making new music, it's clear that Portishead will never quite be an angst-free enterprise.
"It [making music] generally is hell because we're not a band in the traditional sense," explains Barrow. "Our musical set-up and history are really odd."
The group were brought together by Barrow in 1991, a trio from three different backgrounds representing three different musical styles, and at three very different stages of life. Barrow was a 19-year-old hip-hop obsessive whose idea of happiness involved "smoking fags, sleeping on floors and making beats". Utley was a jazz and R&B session guitarist in his mid-30s, while Gibbons was a 26-year-old singer-songwriter and veteran of the pub circuit.
Portishead now seem a less unlikely team than they used to. Barrow is now 39, with two kids, and though he still takes pleasure in delivering colourful rants at the state of the world, he also cites a 1am Pimm's-drinking session as a feature of his recent Glastonbury hedonism. Utley is well into his 50s, large and comfortable and capable of surprising moments of emotional frankness. But even from the start, they understood each other in a way that transcended their differences.
"Geoff has always been one of the few people I talk to creatively in a certain way," explains Utley. "When I met him we vibed on each other even though there was an age difference and musical difference, and that hasn't really gone. Portishead is a world where I know exactly what's going on, and somehow it's almost like it's the real world and everything else is playtime."
There's something genuinely touching, meanwhile, in the way Barrow talks about Gibbons – a reminder of how young he was when Portishead began, and how much his relationships within the band have shaped him in the half a lifetime since.
"Working with Beth has been more a life experience than a musical relationship," he says. "For many years, I was Bobby Beats, smoking 40 fags a day and sitting around a sampler. And she couldn't be any more different from that. She opened things up for me. She has this amazing honesty, and sometimes that's all you want from someone, but it's taken me until my mid-30s to realise that. Before it was all about the music, now it's more about people."
Gibbons remains the gap at the centre of any Portishead interview. Earlier, she had gone through the ritual of the Observer photoshoot with quiet good humour. "Put me anywhere that makes me look stunning," she says, mock diva-ishly to the photographer. But she has always refused to speak to the press, and this, along with the bleakness of her music, has led to a perception of her as the band's tortured soul – detached from life's banalities by the consuming intensity of her art.
"What, you thought that she was the dark lady who arrives in the dark car and gets wheeled out?" Utley chuckles. "That's interesting… Beth's a day-to-day person: we can talk about bread or how crap it is when you can't park; how awful it is when your relationship ends or one of your parents dies, as well as what washing-up gloves to get. There's a darkness within everyone and she's in touch with that stuff. And that's more interesting to her than singing 'Get up, get on up', which is fantastic when James Brown does it, but I can't imagine her doing that."
In the three years since Third, Portishead have released just one new track, "Chase the Tear". They have never stopped working on their own musical projects – soundtracks, collaborations with other artists – but plans for a new record are, to put it politely, at an embryonic stage.
Barrow recently bumped into Thom Yorke at Paddington station, where the Radiohead frontman told him the band had just finished their latest album, The King of Limbs. "And I went, 'Fuck off! Fucking hell, man, in the time you've done another record we can't find two notes that we like that we can put together.' And we can't. And then we get really scared of trying it."
Outside the studio, however, ideas and influences are beginning to emerge which are helping the band to think creatively again. Utley begins to explain what currently inspires him, but he can't get far without cringeing at how it will sound on paper.
"Scandinavian folk music… that makes me sound like a pretentious arse… Certain electronic stuff, some Arabic music. That sounds really shit when I say that… politics, world issues. I sound like such a vacuous model, don't I!" He perks up when describing the line-up for the I'll Be Your Mirror festival (an offshoot of the Butlins'-based cult music jamboree ATP), which the band are curating in London this week and again in New Jersey later in the year.
"If someone says we're going to put on a festival and you can have whoever you want, and then you think: 'Shall we get Grinderman and PJ Harvey?' How brilliant is that! I made myself deaf listening to the Polly Harvey record recently. I listened to it twice full knacker in the car and nearly deafened myself."
Barrow, meanwhile, is finding inspiration less in music than in the current mood of protest simmering across the country: December's student marches and, more recently, the riots that broke out in the Bristol district of Stokes Croft after a police raid on a squat linked to an anti-Tesco campaign.
"People feeling they need to do something, rather than just the apathy of doing nothing – that alone is magical. Anger is crucial for me, and music has always been an outlet for that – even Dummy. 'Machine Gun' for me was just to try and shock people, to go: 'Have some of that! Just get some fucking distortion into your brain.'"
Barrow knows this doesn't square with everyone's perception of his band. Nor does it square with the current scene: a sunny pub garden in a semi-rural corner of Bristol; Alfie scratching himself under the table; sheep cropping the grass down the road.
"Being a 40-year-old guy and being in Portishead, why should I be spurred on by the idea of people smashing things? I have two beautiful children, I live in a nice house and I drive an Audi. I'm most probably the kind of person whose window they want to put a brick through."
And yet it does begin to answer why exactly the three members of Portishead feel compelled to leave their kids, kitchens and the houses that Dummy bought them and plunge themselves back into a process that has caused them so much turmoil.
"When things like 'Machine Gun' happen, it does something to us," says Barrow. "It gives us a rush of achievement and worth. And we haven't got a choice. That's why we didn't do anything for a long time, because we didn't feel like we had it in ourselves to do it. And there's no point saying anything if you haven't got anything to say."
So do they have something to say now? Do they feel like there's the potential for another "Machine Gun"?
"What? In the band?" says Barrow, brightening again. "Yeah! Absolutely. Yeah… I mean, don't get me wrong but, without wanting to be cocky about it… Yeah!"