It was the night of Monday 1 May in the cavernous indie club that is Koko in Camden Town, and Disco Dave Cameron, leader of the Conservative Party, was in the house. The occasion was The Big Ask Live, a benefit concert in aid of Friends of the Earth's campaign to persuade the government to enact a new law on climate change.

Thom Yorke had been doing his bit. The Radiohead frontman and his guitarist bandmate Jonny Greenwood had agreed to break two years of gig-silence to headline the show. Gruff Rhys from Super Furry Animals was on the bill, as was folk singer Kate Rusby, while curly-headed TV pop presenter Simon Amstell was the compere. Not that it mattered. The 1,000 people who had snapped up tickets for the charity event - some gladly hornswoggled to the tune of £150 by eBay scalpers - were only there to see the guys from Radiohead.

Yorke, an 'ambassador' for FoE, had written to the leaders of the three major political parties, inviting them to the gig. 'Well, obviously I didn't write to Tony,' the 37-year-old singer said. 'I wrote to Gordon Brown instead.'

'Obviously' because Thom Yorke hates Tony Blair; because he thinks the PM has 'no environmental credentials'; because Yorke is viscerally opposed to the Iraq war and to current global trade practices. And because the rock star had already declined an invitation to meet the Prime Minister last September.

Why miss the opportunity to lobby the chap on the political throne? Given his passionate espousal of these causes, isn't it Yorke's duty to at least engage in a conversation?

'Not when there were all sorts of conditions being put up.'

Such as?

'[Blair's advisers] wanted pre-meetings. They wanted to know that I was onside. Also, I was being manoeuvred into a position where if I said the wrong thing post-the meeting, Friends of the Earth would lose their access. Which normally would be called blackmail.' Yorke flashed a humour-free smile.

So Yorke wrote to Chancellor of the Exchequer Gordon Brown instead, and Brown (says Yorke) said he would send Environment Secretary David Miliband. The singer's letter to David Cameron, meanwhile, elicited an eager response. The Tory leader wrote back, Jim'll Fix It-style, raving about Radiohead's 'Fake Plastic Trees'.

'I sent this rather sad letter saying I'd love to come to the concert, thank you for asking,' Cameron told Sue Lawley on Desert Island Discs four weeks after The Big Ask Live. 'PS: please play this, my favourite song, and he did.'

Sadly for the rather starstruck Tory leader, Yorke's PR subsequently issued a statement denying this causal link. The choice of songs on the setlist had 'nothing to do with any special guests'.

After the Koko show, the VIP mingling. Yorke met the politicos. He found that both Miliband's and Cameron's wives were 'big Radiohead freaks, so that was quite interesting'.

What did he make of Cameron?'

'He looked very nervous. As you would.'

A politician Yorke could do business with?

'Nnnooo,' said Yorke with pained emphasis. 'But then it's not my job to do that bit. I wouldn't do that with any of them.'

Blackpool, 12 May 2006

Radiohead had just completed the first of six indoor UK dates, in Blackpool's Empress Ballroom. It was a sunny Friday night by the seaside, and the atmosphere was spicy with sweat, lager and marijuana smoke. The set had roared into life with the furious funk of 'The National Anthem'. Back playing in a more intimate environment, it was a trenchant reminder of how great Radiohead are as a jump-around rock band.

Afterwards they, their management, the CEO of EMI, road crew and friends stood drinking beer in the ballroom's basement. The following morning an announcement was being sent out via Waste, the fans' information service on the band's website. It was a message from Yorke. It was mostly in lower case, and the punctuation was skewy.

'this is just a note to say that something has been kicking around in the background that i have not told you about. its called The Eraser... i wrote and played it... it was fun and quick to do. inevitably it is more beats & electronics... no its not a radiohead record. as you know the band are now touring and writing new stuff and getting to a good space so i want no crap about me being a traitor or whatever splitting up blah blah... this was all done with their blessing. and i don't wanna hear that word solo. doesnt sound right. ok then thats that.'

The Eraser - indeed it's a solo album - began sputtering into life when Radiohead 'stopped' two years ago, Yorke told me, after the completion of the year-long Hail to the Thief tour. They had flown the 'wrong way round the world', east to west. 'You're not built to do that. It just spun our heads out, man. I don't think anybody really slept for, like, three, four weeks. So that level of sleep deprivation and doing these big shows under lot of pressure... It was just messed up.'

In 2004 being Radiohead 'was getting boring and it just got a bit weird and self-perpetuating... It felt like everyone was under obligation to do it rather than because we wanted to do it. And one of the things I had wanted to do for ages was get stuck into a bunch of things that I had been mucking around with that didn't fit into the Radiohead zone.'

He explained that The Eraser was 'an accumulation of really sketchy ideas that were going around since I learnt how to use the laptop properly.' It's an insidious collection of skittery beats and pattery rhythms and minimal post-rockisms recorded with regular Radiohead producer Nigel Godrich in the band's studio in Oxfordshire, in Yorke's second home 'by the sea', and in Godrich's studio in London's Covent Garden.

Radiohead endured convulsive - OK, argumentative - times trying to make Kid A and Amnesiac and escape the long shadow cast by 1997's hugely successful and influential OK Computer. Crudely speaking, it was out with the prog rock of 'Paranoid Android', in with the tech-jazz of 'Everything in its Right Place'. Making that transition was a long, bumpy, tortuous journey.

So it's not too much of a stretch to imagine it's a relief to the other members of the band - Jonny Greenwood, Colin Greenwood (bass), Ed O'Brien (guitar), Phil Selway (drums) - that Yorke has found a new outlet for his twitchy creativity. As he told me at one point: 'I was excited about the idea of just using beats and stuff and some of these sounds that I had. And writing to that rather than writing to good old-fashioned acoustic instruments [and the idea that] it's not a song unless it's got a fucking guitar in it or whatever nonsense.'

The Eraser is Yorke doing things his way. The title is, in part, a reference to him rubbing out the weight of his own musical past. Instead of a single, the album is being trailed by the inclusion of the song 'Black Swan' over the closing credits of Richard Linklater's mind-bending animated sci-fi film A Scanner Darkly. Yorke may get his 'arm twisted' to do some solo shows but he doesn't seem convinced it's a good idea.

And the record is being released by big-booted indie label XL (home of the White Stripes). How does that bode for Radiohead's future, given that they have completed their contractual obligations to EMI and are currently without a record deal?

'It didn't feel right to do it with EMI. It was done with the doors shut. As Nigel said, without anybody watching. And it was done in a different context so it felt like it should be put out in a different context. Which is not saying that we won't put things out through EMI or whatever. I just don't personally feel that we owe anybody anything. I think that's a mistake.

'Will we re-sign to EMI?' he mused. 'I don't know. I don't think we'd sign sign to anybody. Give someone a record when it's done if we feel that they can do it justice. That's it.'

Oxford #1

In Thom Yorke's words, I've really 'freaked [him] out' with our 'heavy' conversation. He has gradually shrunk towards the table in the quiet dining room, all furrowed brows and rubbed temples. Now, belting his black raincoat, he bows his head and rushes off out the door of the drowsy Oxford hotel, into the pelting rain.

What has messed with his brain is not the discussion of the sound of The Eraser ('it was very, very deliberately just me and Nigel using computers a lot, just for their speed and a different sort of aesthetic'). It's the talk of its political content. That, and the swirl of obligation and vilification that invariably converges around any entertainer who speaks up.

I had asked him why Radiohead weren't involved with Live8.

'Because that was the point where we couldn't work out whether we should be carrying on or not. Or... well... we couldn't really get it together.

'Also, I didn't agree with the idea.'

Why not?

He weighs his words carefully. 'Because it was a form of distraction. A convenient political sideshow to what was probably the most important G8 meeting... Holding a big rock concert and reducing the issues to bare essential levels, I think, ultimately, was to the detriment of the [Make Poverty History] campaign.'

Did you share Damon Albarn of Blur's opinion that it was inappropriate that it was a nearly all-white bill?

'Absolutely. Damon was spot on. He's braver than I am.'

I read him the quote Albarn reportedly gave to the Sun last month. 'Radiohead - I'm not gonna get into anyone, but bands who care about certain things and then go on one-and-a-half year stadium tours are just total hypocrites... In one sense you've got this developing humanist thing... Then you're creating these massive impersonal events where you're set up as the subject of thousands of people's adoration. Where is the humanity in that? That's just idolatry.'

Yorke considers this.

'That's a bit confused, isn't it? OK, yeah, you're probably right, Damon, I should stop,' he says sarcastically.

Do you feel hypocritical playing big gigs?

'Yup!'

He's never been one for ego or idolatory, so I ask him if it's because of an arena gig's environmental impact, its carbon footprint.

'Yep!'

Seriously?

'Yep. Absolutely.'

So how do you fix that?

'Fuck knows.'

The more we talk about pop and politics, the tenser he gets. To be fair, it's a terrible day: the mother of Radiohead drummer Phil Selway died two weeks previously, and today is the funeral. Yorke could be forgiven for having a lot on his mind.

Nonetheless, he seems not to have thought through how this most polemical of albums will be viewed and discussed. The Eraser is suffused with Yorke's concerns about environmental meltdown - the title is also a reference to the inexorable force of rising tides. The sleeve image is of a King Canute figure trying to hold back a giant wave.

But there seems to be a song with even more sobering content than 'And it Rained All Night', which expresses the fear of catastrophic flooding. Is the song 'Harrowdown Hill' really about the suicide of weapons inspector and government scientist Dr David Kelly?

'It is,' says Yorke with some reluctance. 'But I've got this thing where I don't want to make a big deal out of that because I'm very sensitive to the idea of digging up anything that the Kelly family...'

As he often does, Yorke lets a sentence fizzle out just as another barges in. His speech, like his actions and his responses to issues that he cares about, is impulsive and scattershot. Or, as per the title of the song on 2003's Hail to the Thief, 'Scatterbrain'.

'I don't really think it's appropriate for me to say, "Yes, it's about that",' he continues, 'because I'm sure they're still grieving over his death.'

But Harrowdown Hill is the name of the Oxfordshire woods where Kelly's body was found in July 2003. I remind Yorke of the lyrics: 'You will be dispensed with when you've become inconvenient... up on Harrowdown Hill... that's where I'm lying down... did I fall or was I pushed...'. That's quite direct stuff.

'It's the most angry song I've ever written in my life,' he nods grimly. 'I'm not gonna get into the background to it, the way I see it... And it's not for me or for any of us to dig any of this up. So it's a bit of an uncomfortable thing.'

This is what Thom Yorke, conscious rocker, is like. He's more confused student than celebrity spokesman. More pub ranter than soundbite-spewing talking head. He's more like most of us, in fact. No wonder Radiohead's 23 million album sales and staggering worldwide success come underpinned by hero-worship of Yorke. Fans can relate to him in a way that they never can to superhero Bono and Hollywood-affiliated Chris Martin.

Offstage, Yorke's is a quiet life, and he works hard to keep it that way. He lives in Oxfordshire, where he grew up, with his partner Rachel. They met while both were students at Exeter University. She's a lecturer (for her PhD she studied Dante). They have two children: Noah (five) and Agnes Mair (18 months; The Eraser is dedicated to her). In terms of 'personal detail', you won't get much more out of him that.

Nor will you find him out and about at gigs or parties or openings or any of the usual stuff that's rockbiz catnip. But he will turn up at RAF Fairford to protest the beginning of the Iraq war; or at Westminster for a Trade Justice Movement rally; or at a CND anti-Star Wars demo in Yorkshire. Or he'll perform at an all-night vigil in Westminster Abbey in support of the world's poorest workers.

When Yorke feels things, he really feels them. He's vigorous and passionate, and far from the dour grump he's often portrayed as. It's just that he gets ultra-stressed by things that many of us choose to ignore, and melodrama can ensue. As he said to the NME earlier this year, the preamble to his abortive meeting with Tony Blair made him 'the illest I'd ever got... I got so freaked out about it'. He also told the magazine that the music business is 'such a bunch of fucking retards as far as I'm concerned', which is patently cobblers. When the NME splashed this quote on its cover, Yorke was not impressed and at present isn't talking to the magazine.

I try another, less personal tack with regards to 'Harrowdown Hill'. Did the Kelly affair crystallise everything that was wrong and venal about the whole Iraq adventure for Yorke?

A pause. 'Um, I guess I didn't see it in terms of Iraq, but obviously, yes. What disturbed me the most about it was the way that the Ministry of Defence in this country is able to operate. I think it's a profound cancer at the centre of this society.'

Thom Yorke exhales heavily.

He notices the time. He's got to go. Phil's mum's funeral. He promises he'll come back afterwards and talk some more. If his head's heavy now, what's it going to be like later this afternoon?

Wolverhampton, 15 May 2006

At the Civic Hall, Radiohead continued to showcase more new songs. 'House Of Cards', which recalled Fleetwood Mac's 'Albatross'. The broken hip hop beats of '15 Step'. 'Nude', a fabled song among Radiohead fans that the band have never managed to record. Yorke thought they'd finally nailed it, but then added that 'that's under debate as well...'

The purpose of these shows, he said, was partly to 'road test' the new material, as Radiohead had done on a pre-Hail to the Thief tour of Portugal and Spain in summer 2002.

Then he admitted that the tour was also about Radiohead getting back the confidence they'd lost during the Hail to the Thief 'zone', 'because what we were doing was becoming routine. It felt like we were doing it 'cause we didn't really know what else to do.' Undertaking the brief, between-album 2006 tour gave them 'something to focus on before we went back to recording'. Over the past year the band had 'spent too long in the studio with things not happening and it was getting frustrating.'

The Eraser song 'Atoms For Peace' is about Yorke grappling with his worrywart, paranoid-android tendencies. 'No more going to the dark side with your flying saucer eyes,' it begins. 'No more talk about the old days, it's time for something great.'

'Quite a personal song, really,' Yorke sniffed. 'Trying to correlate my life with choosing to do this, and choosing to get over the fear which is a constant thing I have. Being a rock star, you're supposed to have super-über-confidence all the time. And I don't.' A pause. 'And it was my missus telling me to get it together basically.'

Oxford #2

The woodcut-style sleeve of The Eraser is the work of longstanding Radiohead collaborator and friend Stanley Donwood. Yorke and the artist first met at Exeter University, where Yorke studied English and Art. Donwood's first impressions of Yorke: 'Mouthy. Pissed off. Someone I could work with.' They're frequent partners in graphic crime (Yorke using the alias Tchock or Dr Tchocky), and in the run-up to the making of The Eraser had been looking at 'old German Expressionism stuff, as you do'. The layman might think they'd been looking at Captain Pugwash or Noggin The Nog cartoons.

The Eraser, Donwood tells me via email, 'can be seen as the environment we live in, a Gaia-like force that doesn't care about us, that can sweep away our accomplishments in the space of half an hour.'

Donwood says he had one of his books of German Expressionism stuff - The Nuremberg Chronicle, published in 1493 - with him 'during the flood at Boscastle, Cornwall in 2004. The terrifying sight of buildings torn apart, trees ripped from the earth and the endless thundering roar of the flood remained fixed in my mind. The next day I began to draw, half-copying the woodcuts from the Chronicle, half exorcising my memory. That summer I carried on drawing imaginary medieval disasters, most of which were spattered by the persistent rain.'

Donwood's mention of Boscastle chimed with something Yorke had said: 'One of the initial experiences as we started making the record was [Stanley] and I ended up being involved in this mad natural instant which had a big effect on us.' It seems that Yorke and Donwood happened to be in the West Country on the day of the Boscastle flood.

There can be little doubting Yorke's commitment to the environmental cause. Certainly it's helped him make a solo album that is quietly yet vividly powerful. No dilettante side-project of the idle millionaire rock star, this. But, ever fretful, he knows he could do more. Nine switched-on songs do not an angst-free singer make.

'When I did the launch thing for [Friends Of The Earth's Big Ask campaign], after being up from seven in the morning doing interviews, I ended up in front of Jon Snow and he was like, "So what are you doing?" I'm like, "Not enough". And it was a painful silence in the [TV] studio! You're not supposed to say that. You're supposed to say, "I'm doing this and this and this. I'm planting trees, somewhere, probably." I'm not! I'm not doing enough! None of us are.'

Do you have sympathy for Chris Martin: very hand-on-his heart active in the Make Trade Fair campaign but necessarily lives a bit of a Hollywood lifestyle, and drives a big car?

'I don't drive a big car, I'll give you that. Um.' A pause. 'No one's going to come out of this dirt-free; I don't come out of it dirt-free. It's basically [about] having to make a decision whether to do nothing or try to engage with it in some way, knowing that it's flawed. It's convenient to project that back on to someone personally and say they're a hypocrite. It's a lot easier to do that than actually do anything else. And yeah, that stresses me out, because I am a hypocrite. As we all are.'

Do you have solar panels or a mini-wind turbine at home?

'I don't have a mini-wind turbine because as far as I can work out, they're trying to get the bill through the Commons to have or have not [sic] permission to put them up. They're trying to define them in the same zone as Sky dishes. And things like that. And no, I don't have solar panels yet because I've just moved house and I'm working out how best to do it.'

London, 19 May 2006

Thom Yorke had a few drinks tonight. He'd kept himself straight for the UK tour. But this second Hammersmith Apollo engagement was the last show. It had gone well. He could afford to let his hair down. Now he bopped chirpily through the throngs of folk at the big end-of-tour piss-up. Gnarls Barkley, Siouxsie Sioux, the Mighty Boosh and Jamie Oliver, among other guests, shared his enthusiasm. Radiohead, one of the biggest British bands of the Nineties and Noughties, were back. Well, sort of. Yorke admitted he was frustrated at the length of time Radiohead were taking to record their seventh album. They haven't even settled on a producer yet. They'd started producing themselves, had done some sessions with Mark 'Spike' Stent, and had been speaking to Nigel Godrich and 'some other people'. They had played 11 new songs on the tour, plus their contribution to the latest War Child album 'I Want None of It'.

'It seems crazy to have this all [new material] sitting around... It's to varying degrees finished, [and] to just have to wait for another six months, eight months, seems nuts.'

But when you can, as Yorke can, happily 'noodle for England' it's easier to take your time. And if you're frightened of your carbon-spewing rock star shadow, a recording studio's a good place to be.

Oxford #3

I worry that Thom Yorke might not return after the funeral. But he does. He's fairly upbeat, considering. He apologises for 'his brain not engaging' this morning. 'In some ways I feel I didn't answer properly,' he says, eating an asparagus risotto in the hotel dining room. 'That whole Damon thing,' he begins, then pops on his conspiracy theorist's hat - the Sun printing that quote is another example of 'the Murdoch papers" disdain for him and his lefty, anti-globalisation ways. (There was a savage piece in the Murdoch-owned New York Post once.)

Then he says: 'I'm not really bothered about what Damon thinks, but the whole thing about doing big shows does bother me. Do you just do the small shows and keep selling them out so everyone gets really, really cross [because they can't get tickets]? But at the same time the whole apparatus of big festivals is not cool. If we could go to them and say, you can only use paper cups, you can't use generators, you have to use solar panels... The trouble is you can't do a show at the moment with solar panels. You technically can't make it happen.'

Funny old Thom Yorke. He hasn't learnt to stop worrying and love global warming. He still frets and agonises and spirals into confusion. Good for him. It makes for great, boundary-pushing music. And for heated conversation.

Then again: 'You can spend too long dwelling on stuff that there's no point in dwelling on. The whole thing for me now is just hanging out with Radiohead again. Having done my record there's a point to carrying on working. That's all I need.'

I tell him it's almost 14 years to the day since Radiohead's first single, the 'Drill EP', came out.

'God almighty, is it? Wow!'

Has it taken him 14 years to learn these lessons?

'No, no! We all felt we'd just painted ourselves into a corner. And it all went a bit wrong. That's bound to happen. That's what should happen. What would be more worrying is just carrying on without even realising. It's the same with any art form, hopefully. You just have to stop for a bit out and work out what the hell you're doing.'

What Thom Yorke's doing, he concludes with a strange mixture of a grin and a frown, is 'satisfying a great big need that I have. To hang out with these people, to share ideas, and to make a fucking racket in Radiohead.'

And if he saves the world along the way, so much the better.

· The Eraser is released on XL on 10 July

Yorke notes: Thom's life and loves

Born in Wellingborough, Northamptonshire, on 7 October 1968, Thom Yorke had five operations in the first five years of his life to repair a paralysed left eye - the last leaving him with his distinctive lazy eye. At the age of seven, inspired by Queen's Brian May, he got his mitts on his first guitar, and at 13, he dyed his hair for the first time, flirting with peroxide blonde, black and ginger.

Whisked from school to school by his dad's nomadic job as a salesman of chemical engineering instruments, he finally settled at Abingdon School in Oxfordshire, where along with friends, he formed On A Friday, a band named after that sole day when they were allowed to practise.

The group continued even after its members were divided by cross-country university migration, and despite dabbling with another band, Headless Chickens, during his art and literature degree at Exeter, Yorke stuck with On A Friday. Upon signing, they were steered to a new name by EMI, and Radiohead gasped their first breath.

Yorke has a long-term partner, printmaker Rachel Owen, with whom he lives in Oxford along with their two children, Noah and Agnes.