That afternoon a press release lands in my email inbox. On 2 June EMI Records will release The Best Of Radiohead. It's got all the big singles, insofar as Radiohead have had big singles at all, and a second CD with songs like well-regarded B-side Talk Show Host and their first single Anyone Can Play Guitar on it. But it's pretty far from the present Radiohead way of doing things, and seems to represent a final headstone for the relationship between the band and its old label. It follows a box-set of all the band's albums from last year; it was rumoured that, in a genius stroke of blue-sky new-business thinking, EMI's new owners had originally planned to sell the box exclusively via the retailer that their market research had determined was most favoured by Radiohead listeners: the clothes chain Next.

"We're not really bothered about it," says Thom Yorke with a sigh that suggests quite the opposite. "If they spend a wodge of cash trying to get those songs heard again, then great, but our management tried to tell them that people don't really buy greatest hits any more. Only in Britain, nowhere else. iTunes has seen to that. You might not make your money back. And we haven't really had any hits, so what exactly is the purpose?

"But there's nothing we can do about it. The work is really public property now anyway, in my head at least. It's a wasted opportunity in that if we'd been behind it, and we wanted to do it, then it might have been good."

Are you over your divorce with EMI yet?

"It wasn't an unfortunate divorce," Yorke counters. "I was quite happy to have an excuse not to get involved again. We wanted to be reasonable; we wanted to play along because, technically, they own all our work and if we walk away they can do what they want with it. So we thought maybe we should keep talking to them. Personally, I just wanted to forget about it. It didn't feel right. And now it's like when you move house: you don't want to peer through the window and see what they've done with the wallpaper because it will only upset you."

This is a little disingenuous. There hasn't been a band-label split as bitter as this in recent times. Radiohead's departure from Parlophone - as well as those of Paul McCartney and The Rolling Stones, the latter from EMI subsidiary Virgin - began to be seen as proof that EMI's new owners Terra Firma could not understand its artists. Terra Firma CEO Guy Hands then told the press that Radiohead had demanded "an extraordinary amount of money" -  £10 million - to re-sign with the label; details of the alleged losses that previous Radiohead albums would have made under such a deal were leaked to the business pages. In fact the sticking point in negotiations was continued ownership of the band's back catalogue, and indications are that Radiohead were well along the road of discussions with XL, the eventual victors, as well as a third indie label. Nevertheless, Radiohead were now on the receiving end of the aggressive language of mergers, acquisitions and corporate raiding; ironically, the subject of more than a few of their own songs over the years. How did Yorke feel about being briefed against as if he and his management were rival CEOs?

"It fucking pissed me off," he snaps. "We could have taken them to court. The idea that we were after so much money as stretching the truth to breaking point. That was his PR company briefing against us and I'll tell you what, it fucking ruined my Christmas. I was so angry, I decided I'd go for a walk, come home, write something on Dead Air Space [Radiohead's blog] and that would be it, it's over, the end. There was a certain satisfaction in knowing that I could redress it on our own website, but it was a clear indication that the relationship was over."

We're talking in a secluded upstairs room of a hotel in Covent Garden. Thom's curled up at the end of a chaise longue, drinking tea and wearing a new black Fred Perry shirt (good choice, I tell him; he seems a little startled and looks down at the logo as if he's noticing it for the first time). In person Thom's not as bunched and intense as he appears onstage -  who could be? -  and his sense of humour is self-deprecating. "We have to go through the misery to get to the joy," he says, grinning. "You don't want to spend too long in our world, I tell you."

But Thom's turn of phrase does sometimes mirror his famously abstruse lyrics. Lobbyists and political middlemen are a wall of "black energy"; the recording of In Rainbows dragged on so long that "we all started to think that none of it had ever happened, that we'd never been in a band, it was all a big Truman Show gag." We talk for a while about how he writes them. True to form, it's a painful business. He fill notebooks with potential lyrics but finds he uses maybe five per cent, or maybe just a single line from each book. He can't even write the words unless he can physically play the songs to himself. When he made his solo album The Eraser in 2005/6, he and Godrich had completed all the music in the form of loops and samples, but he couldn't finish the lyrics. So Thom had to learn to play all the songs by hand, on guitar and piano, before the words would come.

He must be aware that there are thousands of people scanning his lyrics for hidden meanings. Are they just trying to decode the undecodable or panning for actual gold?

"Yeah, fool's gold! It's not a very musical way of looking at things, to sieve it for meaning. It's more about feeling it, isn't it? If you write lyrics intending for a set meaning to come across, it's not art, it's just a rant. The best lyrics come from a particular moment when you've written a piece of music or you've learned a new way to play, or do something with sound."

He's now known as much as a political figure as a frontman, and this is something that can cut both ways. For everyone who takes on board what he has to say about Friends Of The Earth's Big Ask campaign for mandated carbon reduction, there'll be someone who wonders if there isn't something a little simplistic and student-y about his politics. See, for instance, the cartoons of businessmen with fangs on the artwork for Amnesiac, which Thom prepared with collaborator Stanley Donwood; the "kicking, screaming Gucci little piggy" of Paranoid Android; or Hail To The Thief's title. Did we really need a trite pun to tell us there was something fishy about the American presidential election of 2000?

To be fair, Yorke is absolutely sincere in his beliefs - Ed O'Brien had told me that climate change had been a big thing of Thom's since he had come to visit Ed at Manchester University nearly 20 years ago ("He wasn't exactly running around turning the lights off," Ed said, "but it was quite a far-sighted discussion. He was a bit ahead of his time"). Thom was surprised at how uncynical most MPs have turned out to be - it's the intermediaries you have to look out for, he says. And Radiohead are certainly walking the walk with their low-carbon tour. They even looked into touring the US by rail, "but America doesn't have a national rail network like they had in the fifties. I want to do the Some Like It Hot thing - travel the country on train bunks." Thom reads widely, pays attention, scowls a little when I mention Nick Cohen. He knows what he thinks. He is also aware that there's no target for stick like a rock star who talks politics.

When Friends Of The Earth asked him to front The Big Ask, his first response was to say, "You must be fucking joking. Me? The last person in the world. It's a bit like saying, 'Everybody stop flying, except us.'" Then he thought about it and concluded that any personal grief he might suffer would be worth it. "The tough aspect was having some twat from the Sunday Timesfollowing mnee around and pestering my friends to learn that, guess what, he flies in planes sometimes! But FoE wanted me involved because I wasn't holier than thou. None of us is, we live in a carbon society. The subtext is: if you have a political establishment that is so far up its own arse that it demands that someone like me, of all people, has to engage with a subject like climate change in order to get it noticed, God help us all."

Sometimes Thom will be in the middle of a politically-oriented interview and he'll think, "Oh, just shut the fuck up will you?" Whatever he says will always come across the wrong way to some people; folk going about their daily lives don't need some guy in a band giving them a hard time about their carbon emissions. I ask him if musicians ever come out of politics well?

"No."

Does that bother you?

"Yes. Yes, it does. But it's only my arse. Sometimes you have to participate in the things you agree with and accept that some people are going to hate you for it." Radiohead tend to work on "Thom time". He will be the one who decides the band needs to tour, or take a lengthy break from one another - and this, he accepts, creates the terrible problems with getting back to work which he himself is now sick of. He has in the past sent up his role as Radiohead's de facto ruler, taking on the personae of the tinpot politicos he despises in songs like Sit Down. Stand Up and mocking himself in squibs like I Am Citizen Insane. Does he worry that these songs might have a grain of truth in them? He squirms a little.

"Nnnnnn... I kind of have to stay true to what my instincts are. The good thing about everyone else in the band is that they will tolerate it. Sometimes I resent being the point person - as they say in the corporate world - or the mug, as everyone else says. I was talking to Michael Stipe about the difficulties of being the frontman and the main spokesman and all the rest of it and he made a really good point, which was, 'Yeah, but they have to put up with us. That's hard.'"

Read Part Five.