David Cameron is taller than I expected, posher than I knew anyone who isn't a Royal is allowed to be, and (when I catch my very first glimpse of him) somewhat agitated. He pulls up into the driveway of his constituency home - a rather nice converted barn, in Dean, just outside Chipping Norton, Oxfordshire - as I pull up in a cab on the road outside; and now he's worried that my cabbie's about to reverse into the pristine grass verge borders of his neighbour's garden. He's jumped from his car and is gesturing at my cabbie, crossly, and issuing terse, loud instructions - your classic posh man caught in a disagreeable situation. My cabbie meanwhile, is oblivious to the crossness - and growing increasingly excited.

'I know him! I know him!' he says, breathlessly. 'I know his face!'

'Yes,' I say, watching Cameron as he approaches, fiercely.

'Who is he?'

'He's David Cameron, the leader of the Conservative Party.'

'That's right!' he says, and (driven either by political zeal, or celebrity lust, or perhaps a combination of both) he launches himself from the driving seat, directly into Cameron's stompy trajectory. It takes Cameron microseconds to adjust to the fact that the small Asian man who was threatening his neighbour's lawn moments earlier is now bearing down on him in a heightened state of giddiness, and then he's there - recognising a constituent when he sees one, shaking the man's hand, accepting his good wishes, smiles all over his very large, very pink and scrubbed face.

'Hello, hello!' Cameron says. 'Good to meet you! Very glad to meet you! Hello!'

He turns to me. I'm fumbling around, trying to detach myself from the back of the cab and secure a receipt for my cab ride, which isn't easy; my cabbie is still gazing at Cameron adoringly.

'I'm David,' he says. When I finally free a hand, he shakes it. He smiles. Then: 'Oh!' he says, when he notices my handbag, which was designed by his wife Samantha, creative director of Bond Street stationers-turned-purveyors-of-exquisite-leather-goods Smythson, and which is named 'Nancy' after his daughter. He's genuinely pleased to see it. 'Well done, you! Passport to a warm welcome, here. Passport to a warm welcome!'

David Cameron isn't quite sure what we want from him. He loves Observer Food Monthly. 'It's my favourite!' he says. But why are we here?

'We think you're an eco hero,' I say.

'Ha!' he says. 'Are we going to talk about climate change?'

Yes. Climate change and food.

'I like food,' he says. 'I'm very greedy.'

How does David Cameron qualify as an eco hero? He became a poster boy for green in April 2006, when he took a trip to a glacier on the Norwegian archipelago of Svalbard to witness first-hand the physical effects of global warming. He'd been building his image for a while - he set out his eco agenda on his election as Tory leader, in December 2005. 'I tried to make a start this morning by biking to work,' he said, in his acceptance speech. 'That was a carbon-neutral journey until the BBC sent a helicopter to follow me.'

But that tour of a glacier - organised and supported by the World Wildlife Fund - sealed the deal. It generated an iconic image - Cameron, powering hatless through the frozen wastes on the back of a dog sled - and a certain degree of sniping. It was, critics said, a flagrant publicity stunt, a photo opportunity (why else, they asked, was Cameron not wearing a hat in the sled shot, despite obviously being very cold indeed?). Others calculated the carbon footprint for the entire trip - which was sizeable, if offset by (among others) the World Wildlife Fund.

But regardless, this was a turning point for conservationist politics in the UK. With it, with that picture, Cameron and his Conservatives co-opted the environment, and the public (who were in the throes of an Al Gore-inspired awakening) registered their approval in local elections held within a fortnight of the Svalbard tour. The government was inspired to give global warming some serious thought. The Climate Change Bill was eventually passed, in no small part as a consequence of a spate of prolonged political one-upmanship on eco.

Having made green his thing, Cameron ran with it. He appointed high-profile, golden-haired, publicity-luring pretty boy Zac Goldsmith as an adviser on ecological issues in 2005. In 2007, Cameron outbid Labour's commitment to cut carbon emissions by 60 per cent by the year 2050, by calling for an 80 per cent reduction. Throughout, he's done much public riding around London on a push-bike: more photo ops which were only slightly tarnished by the revelation that he needed a car to follow him, to transport his shoes and his briefcase to Westminster.

Having dispatched the star-struck cabbie, Cameron ushers me into his constituency home. It's an artful mixture of fashionable and posh and bog-standard domestic havoc. The porch is a tangle of mud-spattered Hunter wellies (the perfect marriage of fashion and posh, those) and coats that are not quite Barbours, but hint at that sort of thing, and scarves and single gloves and the general accumulated detritus produced by three kids under the age of five. There's discarded Smythson packaging everywhere.

In the marginally less dishevelled kitchen of the converted barn, Samantha Cameron wears excellent pyjamas, and butters toast and entertains her children by bending balloons into animal shapes, (something she does well, even though she insists this is her first time). She is surely the dictionary definition of 'glossy'. Or 'fragrant and swishy'. Or maybe 'Glamorous But In A Thoroughly Sensible Way, Like A Less Slutty Nigella Lawson'.

'Oops!' says Cameron, closing the front door behind us, and swooping up the three empty wine bottles that are languishing on a kitchen surface. 'Thought I'd recycled the last of those, ha ha!'

Samantha's father's there; so is Gaby, Cameron's beautiful aide du jour, who's just come back from honeymoon, and who calls David 'Dave' - as does everyone, it transpires, apart from Samantha, who calls him 'lover' and also 'babe'.

'Look at this!' Cameron says. He's bouncing about, from one thing to the next, like Zebedee of The Magic Roundabout fame, and he's settled briefly on a flashy-looking coffee machine. 'My Christmas present! Isn't it great?'

'For espressos?' I ask.

'Yes! And cappuccinos!' he says.

Then he bounces off to look for 'a sticky, rolly thing', a lint remover, because there are some flecks of something or other on the navy-blue casual-fit M&S shirt he intends to wear for our photographs ('It's one of Stuart Rose's. Although Father Christmas gave it to me'), leaving Gaby to make me a coffee, and so I do not get to experience a Cameron Cappuccino, which is a shame.

He returns, de-linted, and we sit at a big battered kitchen table, with Arthur, the Camerons' youngest son (whom they call by his middle name, Elwen), and we begin to talk ecology and food.

David Cameron had his wake-up call to the significance of green politics, he says, as early as 1989, when he heard Margaret Thatcher speak on the issue: 'well ahead of its time, actually'. Until that point, it had been a quiet, personal passion of his: 'Because I was brought up in the countryside, and I've always had a bit of an interest in nature and conservation and food and the environment. I mean, I live in the countryside, I adore the countryside. The countryside to me is only the countryside if it's a living, working countryside, and not some sort of museum.'

He became leader of the Conservative Party, and 'what I felt is that you have an opportunity, when you're leader of a party, to put the issues you care about, up front and centre. And with the environment it's been sort of particularly effective, because some people find it surprising coming from a centre-right political leader.'

Cameron, I am learning, is never more than a heartbeat away from spouting some well-rehearsed rhetoric; on this occasion, about how very, very excited he is about the role he has played in marrying right-wing political sensibilities and ecological concerns. He's excited by Sarkozy's conservation efforts too, and also by Arnold Schwarzenegger. He's talked climate change with Governor Schwarzenegger, indeed! Was that really a little bit odd, I say. 'It was great!' he says. 'Yes! It was, er ...' he says.

Odd, I say?

'Oh, no ... What's odd ...' he asks. 'He actually is a very effective politician. One measure of his effectiveness is that he was re-elected, and that's always a good way to measure things! Ha!'

And then he reverts to his thoughts on the surprising-yet-actually-not-surprising-when-you-think-about-it union of centre right politics and ecological concerns, and buzz words like 'stewardship' start flying about:

'I think if you're on the centre-right, it's all about passing on things to future generations, holding the planet in trust. All of that sort of language of stewardship, comes very, sort of naturally to a Conservative politician.'

As does upping the game for the government ...

'I think we have upped the game for the government ... And I don't mind if they do good things as a result of the pressure that we put on them - that's great! And if they get some of the credit for it - or even all of the credit for it - then, you know, who cares?'

I don't believe him for a moment - I think he'd care very much indeed if the public began associating an ecological message with Gordon Brown rather than him; and that he also knows there's little chance of that, which frees him up for some theoretical generosity.

On a personal level, David Cameron and his family lead the sort of semi-green existence that defines most of us now. A bit hit-and-miss. A bit confused. Guilty. Good-hearted. Embracing with relieved relish the bits of it that overlap with a good lifestyle (he's been spotted perusing the glamour-goods on sale in the Cotswold flagship branch of Daylesford Organic, Britain's most-fashionable farm shop, for example). He recycles ('It's sort of a learning curve, you know, for everyone'), and he feels guilty about the amount of flying he does.

He's got a vegetable patch.

'I have, and it's a total mess at the moment. It's winter so there are just a few mouldy Brussels sprouts, and the one red cabbage I haven't eaten, and that is about it. I do very good red cabbage, this year it was a real success!'

Sounds fabulous, I say.

'I love my vegetable garden, ha ha! I know! It's a really nice place to go and sit and contemplate the world. And get away from it all.'

Is it therapeutic?

'It is quite, yes! Because if you want to do something that gets work out of your head, then ... digging! It's good, because you've got to concentrate a bit, you know, on the digging ...'

He pauses, and looks over at Gaby, who is making 'careful, now' gestures at him, from somewhere beyond my right shoulder.

'Oh, stop making faces at me!' he says.

Gaby harrumphs, suggests that he maybe shouldn't dwell on the vegetable patch. Later, she'll tell me she thinks I really shouldn't use the pondering in the vegetable patch bit. Not because she doesn't trust me, she says. It's other people. She can just imagine the headlines.

'Go on!' Cameron says, impatiently.

I ask him if he thinks there's a danger of a mood of green puritanism, of smug middle-class Britain slagging off its neighbours for their un-eco ways.

'I think there is,' he says, 'and look, I've been, sort of, had my bins searched, by a national newspaper and, you know, not every nappy was recyclable! And no one's perfect, and no one should pretend they are! And obviously, in my job, I have to fly a lot. You can look in the register of public interest and you'll see that I often have to use helicopters, so I don't think we should use this as a hair shirt. Trying to beat everybody up with it. I think we all need to play our part, but we need a framework within which that becomes sensible and possible. Some people on the left believe that environmental changes will only come with a really active government telling everybody what to do. That's not true. Some of the hair-shirted environmentalists say we'll only achieve this if we all put on a hair shirt and stop breathing ... and actually the truth is, that what you need is, changes to the system ...' He breaks off and laughs as Elwen staggers into view. 'Don't we, Elwen? - yes, changes that enable people to live greener lives. Recycling's one example. Decentralised energy's another.'

Decentralised energy is Cameron's Next Big Thing, politically speaking. It involves a rethinking of the power system, so that individuals focus more on generating their own energy - with solar panels, with wind turbines, with combined heat-and-energy boilers - and then have the opportunity to sell any surplus back to the central grid. 'You know, because then when you put in your new boiler, of course you'd think: hang on, why don't I put in a combined heat-and-power boiler, so that I can sell electricity back to the grid, and it'll save me money?' He's very big on 'that combination of things, that mixture of altruism and egotism, is never a bad thing! It works.'

Personally speaking, Cameron's Next Big Thing is composting. It's frustrating him currently. 'It's the thing I have difficulty with here. I tried one of those Green Cones, but the water table's so high that, when I dug the hole, it just filled up with water. So I'll have to think of something else. It is quite difficult to get it right. Exactly!'

What else would he like to see, I ask. Vegetable patches for all?

He thinks I'm being silly now.

'Well, no! I mean ...Look ...' (Cameron's a master of the brusque politician's 'Look!' It's a kind of verbal wrist-slapping.) 'Look - if people want to do it, go for it! I do it selfishly. I don't grow vegetables out of the kindness of my heart. I enjoy it, I like eating them, I'm greedy, I love cooking, and it's a good way to unwind.''

We stop after 20 brief minutes, because David doesn't have much time - 'I've already missed a conference call', and he has to get his picture taken. He's nervous about this. Partly because of the food content of the image. He's not sure about being snapped with an egg. He's OK with an apple. I suppose an apple is intrinsically less silly, a bit more classical and biblical; and definitely less laden with Edwina Currie-esque overtones. But he's also worried about the pictures, because he has got, he admits 'a bit of a moon face in some'.

He chats on. He talks about cooking - he likes Jamie Oliver and any one-pot approach; he does most of the cooking, even though Sam's the better cook. Pork belly, slow-roast lamb: 'I'm keen on that, yes.' He talks about buying organic, which he does as much as he can. 'When it comes to meat, I always try and buy free-range ... but you know, like everyone else, if I'm in a rush, and the kids have got to be fed, and all I can find is a pack of bog-standard sausages, then I'll buy a pack of bog-standard sausages.'

He talks about Dave - the lamb he helped to deliver on a nearby farm just after Christmas, who had a twin who died ('so that was sad'); and how, no, they aren't going to eat Dave, they're going to take him out on rallies and make a Tory mascot of him, ha ha! He tells us about the troublesome godparent who bought his three-year-old daughter make-up, of which he doesn't approve. 'It's make-up for little girls, but still!' And he talks with breathless, girlish enthusiasm about the latest BBC adaptation of Sense and Sensibility. 'Isn't Andrew Davies a genius?' he asks. 'I can hardly wait till Sunday!'

And then he's done with us, dismisses us as brusquely as he did the over-excited cabbie, and wanders off to take a phone call.

It seems unlikely that David Cameron, the politician, is entirely sincere in his eco ambitions. Would he dump the whole lot in a heartbeat if it stopped resonating with voters? Actually, that's irrelevant. His critics might claim that Cameron's thrust green sensibilities into mainstream politics for cynical reasons - but he's still done it, and it's still worked. We're all better off for it, more aware, more conscious. More inclined to recycle, more inclined to shop locally and organically, more inclined to demand green responses from Labour. None of this would have happened without him. Meanwhile, Cameron the human is definitely sincerely eco. It's a while since I've heard anyone talk with such passion about his ongoing struggle to precisely balance compost. It's really quite sweet. So for all these reasons, Cameron qualifies as an eco hero.

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