Zac Goldsmith: Not such a green god

"I was horrified," he says now, quiet,sparking up yet another rollie. "I had non-dom status because of my father, but being non-dom has made very little difference to how much tax I have paid. I have always been tax resident in the UK, and the vast majority of my income comes to the UK, where I pay the full rate of tax on it."

Still, David Cameron can't have been too pleased, calling publicly as he did for him to become a "dom". Zac insists he had already given instructions to his accountants "before the current controversy, and not because of outside pressure", and says he is now not a non-dom. "I had always intended to end it before the end of the tax year."

I start to wonder why on earth he didn't sort it all out at the start of his candidacy, but Zac point blank refuses to give me any more information. He has a line and is sticking to it. Anyway, he and Cameron "haven't spoken about the matter", he says, adding that he agrees with the party's policy of imposing a £25,000 levy on non-doms. He is equally evasive when asked if other rich members of the party should be held to account: I mention the Tory donor and power broker Lord Ashcroft, who has persistently refused to clarify his tax status - most recently on Friday, at the behest of Chris Huhne, the Liberal Democrats' home affairs spokesman. "It is a matter for individuals," Zac says.

Anyway, the incident provided further embarrassment on Wednesday, allowing Gordon Brown to score a point at prime minister's questions by bringing up the so-called "Eton problem" - the fact that Zac, Cameron and other prominent Tories are rich Old Etonians. Is there one?

"No," he says, waving his cigarette. "Cameron's got the right people doing the right job, and if it means two or three OEs, so be it. I don't even know how many OEs there are on the front bench," he shrugs.

And if that weren't enough, a speech he gave on Monday night also served to highlight a rift in the Tories between his brand of eco and everyone else's. Admittedly, Zac has always stuck out - a wilful, defiant exotic amid the grey cardboard cutouts - moreover, he hasn't always been a Tory. In 1997 he campaigned for his father's Referendum party, only joining the Conservatives under Michael Howard in 2005.

In spite of suspicions that the editor of The Ecologist was actually still more green than blue, when Cameron took over as leader he was swiftly selected for the A list, joining John Gummer in a policy group to overhaul the party's green thinking. Together they produced a report, Blueprint for a Green Economy. By 2008, however, Zac's light was fading, with Cameron rather damningly describing him as merely "one of many" in the party's environmental team. Was Zac's brand of environmentalism simply too potent for the leader? With his strong views and independent wealth, was he a loose cannon?

That's certainly an impression I get today. Zac is frank about where his green views differ from the Tory manifesto, which he says still contains "gaps, obviously", particularly in respect of what he calls "the car fleet", and what you and I just call cars. "It's one area that you need a blunt instrument, and that is not yet a party commitment," he says. "We need, very simply, to say that at the point where you buy a car, you have a great big new tax applied to the dirtiest ones. I'd like to see that commitment."

Then there are nuclear power stations. Zac is fiercely opposed - how does he square this with the rather woollier Conservative policy not to stand in the way of building new ones, as long as no taxpayer subsidies are required?

Zac claims it would be impossible to put up nuclear plants without handouts: "There has never been a nuclear power plant built without that." What if the Conservatives don't stick to their policies? If they fudge them? Would he resign?

"I don't expect to get my way on every issue. It's a big battle of ideas, and I'm part of that battle of ideas." There are differences of opinion? "On new forms of power," he says, "it's crazy to pretend there's an absolute consensus."

Zac needs a thick skin: he has been much mocked of late as well. For being rich. For being yet another messianic Goldsmith. For his separation from Sheherazade, his wife and the mother of their three children. For his affair with his brother's sister-in-law, Alice Rothschild. For his love of gambling and partying.

I am sure his social profile irritates a lot of people - in the past it has irritated me, too - but now I speak to him properly, I see a well-intentioned if slightly fragile, self-involved Peter Pan, whose most immediate fault, if it is a fault, is that he cannot stop talking about green affairs, his new book, The Constant Economy, and of course Copenhagen. He is not going to the climate conference and is sceptical about its long-term benefits.

"We're so bored with seeing targets set and missed," he says, referring to the shortcomings of the Rio de Janeiro and Kyoto summits. "I don't think they can set particularly useful or realistic targets; I'm not holding my breath for an extraordinary agreement on emissions. I think [Barack]Obama has used up all his political capital on healthcare issues. He'll make a great speech, but it'll be some months before he's replenished his political capital."

If we're unlikely to come up with targets that may be realised, surely the whole thing may be pointless?

"If we come out of Copenhagen without any proper emissions reduction targets, without a proper agreement on forests, then the whole thing's been a waste of time. I think some good stuff will emerge, but nothing like what we need."

Just a terrific waste of energy then?

"I don't go along with that," says Zac. "It's like when people attacked Prince Charles when he went to Brazil. By going to Brazil, he started the process of pushing forests right up the political agenda, so I don't care how he got there."

Talk of princes leads us back to privilege. He admits that he finds it easier to pursue a green lifestyle because of his wealth."If organic food costs twice as much, I can afford to do that," he says, "although I've never pointed to my life as an ecological model. My environmental footprint is big: I'm dependent on the car." He drives a battered old Toyota Prius that he'll keep "until it doesn't work any more".

He splits his time between Richmond and a farmhouse in Devon, where the estranged wife lives with their children, but on this matter, he remains keenly private. When I ask him his views on marriage and family breakdown, an important political battleground at the moment, he goes all tense. Why won't you answer these questions, I say. It's all very well answering on green issues, but as an MP you'll need to speak about wider issues as well.

He sighs. "There are private aspects of one's life that will remain private," he says. "I don't believe ... There is a line. I have never spoken about my private life and I wouldn't. I'm just not going to do that. I said on the day I was selected, if you ever see a picture of my children in my political literature, I will stand down at that moment. I just won't do it. It's fine if other MPs want to do it, but I just won't."

Besides, I do wonder how quick, how confident he is at thinking on his feet. Certainly he wasn't at Eton. He didn't much enjoy it and was expelled at 16, when he was found with marijuana, which he has always maintained wasn't his. Has he ever taken drugs?

"I will answer that: once," he sighs. "Yes, obviously. But that particular time it was a stitch-up. For the first time in my life at school, I was innocent. I wasn't smoking it, it wasn't mine, and I still don't know where it came from. But nevertheless, I took the hit." He sparks up another fag. "Smoking," he says with a grin, "was my first act of rebellion. I will never give it up."

The Constant Economy, by Zac Goldsmith, is published by Atlantic Books