If David Cameron wants to survive to become the next prime minister, he should avoid being driven at all costs. We are hurtling through the narrow, winding country roads of West Oxfordshire, having left his constituency headquarters in Witney (Tory-blue carpet and chairs; wobbly round table; rough Cotswold stonewalls) a fraction behind schedule for the 20-minute journey to Chimney Meadows nature reserve, where Cameron is to deliver a speech.

He is still tanned from his holiday in the Egyptian resort of Sharm el-Sheikh – “Yuh, Tony Blair territory” – and looks slimmer and, strangely, after all he has been through in recent weeks, even more fresh-faced and youthful than when we last met. “Really? I feel a lot older,” he grimaces when I tell him. He is definitely, however, more uptight, barking at the poor chauffeur: “Are you sure this is the right direction?”; “This isn’t the way I would have gone” every few minutes; continually worrying about running late. After our white-knuckle ride, Gabrielle Bertin, Cameron’s press officer, confides that her boss is always a bit of a nightmare passenger. Here is a man who hankers to be in the driver’s seat in more ways, clearly, than the big one.

The occasion, described to me by Cameron as “very birds and trees”, is attended by 100-odd local wildlife enthusiasts in green wellies, Barbours and fleeces; not all of them Tory voters. By the time he is called upon to speak, the leader-in-waiting has regained his customary composure. He starts with a little joke that this is the first time he has given a talk in a barn which means that there’s less risk – what with the fresh air and open doors – of his audience falling asleep.

The speech is an opportunity for him to underline his green commitment: “Some would say that in a recession we can’t afford to go green, but I would say quite the opposite… Some people find the greening of the Conservative Party rather peculiar but…”, and says that he wants to make it easier for local communities to take over disused land, in a move that would echo Thatcher’s “right to buy” homes policy.

He finishes by drawing on the theme of a Dr Seuss book, The Lorax, which is the Camerons’ current bedtime reading for their children, Nancy and Elwen. It is a fable of the dangers of destroying biodiversity: “The effect of deforestation, smog and pollution – gluppity-glupp and schloppity-schlopp to Dr Seuss – are all too familiar… The moral of the story is all in one word: ‘Unless’.” Cameron adopts a singsong voice: “‘Unless someone like you cares a whole awful lot, It’s not going to get better, it’s not.’”

It is six weeks since Ivan, the Camerons’ eldest child, died. David Cameron had agreed that we should meet to talk briefly about this sad event, before our drive to Chimney Meadows. It would be fair to say that we were both rather dreading the intrusion into such a private realm of loss. We had, however, spent months talking on and off for this profile, during which time I had met Ivan in the family’s London home, and the grieving father must remain a pragmatic politician.

First, we catch up on the big news of that week: McNastygate. I’m most interested in what he thinks the whole Damian McBride affair says about Gordon Brown’s character, but the question confounds him: “I don’t know; I’m not a psychoanalyst. I don’t think you can just say that this is a one-off problem. There is a pattern of behaviour but I profoundly believe politics doesn’t have to be like this.” Cue predictable guff on how much healthier the political culture would be if the Conservatives were in power.

I wonder which of the prevailing theories about Brown rings truest to Cameron: is the Prime Minister a Jekyll and Hyde character (donnish introvert alongside bruiser bullyboy) or someone who needs the likes of McBride because he doesn’t have the requisite toughness for that sort of gloves-off fight?

“I just don’t know the man well enough to answer the question,” he sighs. “But you choose the people you want for the skills they have and what you want them to do so… it’s like the scene in Casablanca, as Andrew Neil put it, when the barman says, ‘I’m shocked that people have been drinking in my bar.’ ”

Politics, he agrees, sometimes – quite often – means being unpleasant (as anyone who has witnessed recent Commons confrontations between Brown and Cameron will attest). “Yes, there’s rough and tumble; yes, there’s argument; yes, it can get very heated; yes, sometimes it does go into… um… what people’s motivations are…” And nasty? “Yes, but it’s a world apart from what’s been going on here, (a) because it’s all untrue, and (b) because it’s going after people’s wives. I felt really very angry about it because I thought, ‘How can these people say that about someone’s wife?’” (The leaked e-mails contained below-the-belt comments about George Osborne’s wife.)

Did it make Cameron want to punch him? (I meant McBride but Cameron clearly thought I was talking about Brown which makes his response even funnier.) “You want to get the sort of mud wrestling scene from D.H. Lawrence! No!” he laughs. “I just felt really angry and the point about me saying, ‘I want him to say sorry,’ was not a ploy to be clever. I just thought, ‘I bloody well want him to say sorry. I’m really angry about this.’

“And it’s no good saying he didn’t write the e-mails. This happened in his office in Downing Street, he’s the leader, he’s the head of the culture, so you have to say sorry. He did eventually, and good for him, quite right.”

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