The doors close just as we reach the back of the train. The guard is leaning out of his window. Cameron asks him to open the doors. There are still 45 seconds to go. “No,” says the guard. Bertin, myself, the photographer and, indeed, Cameron all resist the fatal follow-up: “Have you any idea who this is/I am?” Seconds pass. The guard is smiling, taking obvious pleasure in the standoff. It’s just like the good old days of British Rail. The train finally pulls out, leaving the Leader of Her Majesty’s Opposition stranded. An old man behind us is chuckling: “See: Labour made the trains run on time.” Cameron laughs. Bertin is frantically recalculating his schedule.

I step back and look. The hair is very brown and lustrous, though immobile and undisturbed by the running. There are very slight grey flecks over the ears. It is cold but Cameron is wearing an excessively thin three-quarter-length coat over his suit, tie, etc. He is only slightly taller than me, but the overall effect – narrow trousers, precise coat, smooth face, high shining forehead, quiff – makes me feel he is much taller. The phrase “young blade” comes into my mind; he is like a blade, cutting through the air and, somehow, floating a few inches off the ground. Sharp, unencumbered, light, an insouciant Victorian cavalryman, here to add tone to what might otherwise be a vulgar brawl. The unconcern he exudes is massive, natural, inbred. There will be another train, the schedule will be fixed. I, David Cameron, 39, am here, all is well.

And sure enough, magically, the previously dark train springs into life and we pile into first class. “More room to work,” explains Bertin, meaning: “Don’t get us wrong, we are one with the people and not at all snobby or anything, but sometimes, we need, you know, a little space.” Cameron checks in with his wife – it’s just days before she’s due to give birth – and all is well.

But, at last, some of Cameron’s eerie calm has dissipated. In fact, he has become jumpy. He has forgotten his nicotine patches, and he’s five weeks into kicking a 15-a-day habit. Does he want us to know both that he smoked (ie, is human) and that he is giving up (is sensible), or is he just unspun on this matter? Bertin offers to phone ahead so there will be someone waiting with patches at the station. But he says no and apologises in advance if he seems edgy. “Right,” I say, turning on the recorder, meaning: “Edgy is good.”

What are we to make of this man? Six months ago the Conservative party had all but gone to join the choir invisible. Staggering through the interminable leadership battle brought on by Michael Howard’s decision to step down but not actually leave after the last election, the Tories were scarcely able to breathe unaided. None of the contenders – Cameron included – looked remotely plausible, and some – David Davis, Ken Clarke – looked like suicidally elaborate attempts to reprise all the mistakes – narrow focus, Euro-rows – of the past eight years. The young hated them and the old despaired of them. Even large parts of the “Tory” press had decided that a Gordon Brown succession was the best bet.

Then, in October, Cameron addressed the party conference: “So let the message go out from this conference…”

Good grief, he was replaying Kennedy’s inauguration address: “Let the word go forth from this time and place, to friend and foe alike, that the torch has been passed to a new generation of Americans.” Significant, that.

“… a Modern Compassionate Conservatism is right for our times, right for our party – and right for our country. If we go for it… If we seize it… If we fight for it with every ounce of passion, vigour and energy from now until the next election… nothing and no one can stop us.”

Everything changed. Cameron, suddenly, was a contender. More precisely, he was the contender. The runoff against Davis was close but that was only because the penny had not yet dropped among the Europhobe hangers and floggers. If the whole country had been voting, Cameron would have won by a landslide. With his super-plausible bedside manner, he was seen as the bracing detox regime for a sick nation.

This was the point – we felt ill. Cameron was light, shiny, he rode a bike, he looked, well, healthy. Blair has been looking sicker since the war and Brown more sinister and bloated. Even the economy – the one thing that “new” Labour had been able to rely on – had turned strange with productivity plunging, the tax burden soaring, consumer debt rocketing and public services getting worse. The long binge was coming to an end and we needed looking after. Here’s Doctor Cameron.

This essential timeliness of the man was further proved by the fact that, when he did win the leadership, everything changed again. Where once a Brown victory at the next election had looked inevitable, suddenly it was in doubt. In fact, to my eyes, the prospect of lugubrious, heavy, encumbered, tax’n’spend Brown beating light-footed Cameron was absurd. Why vote for Bloaty when you can have Floaty? The Westminster wonks told me Brown would still win, the majority was still too much. But what do they know? Something had changed and it wasn’t just the Tories. It was us. I ask Cameron on the train: was it a generational change?

“Yeah, I suppose that’s right – it’s sort of, you know, when you see pictures of people at university… I see pictures of Blair at university and I definitely think that’s a different generation. Sort of flower-power hair and, you know… I mean, that marks it out.”

Times recommends

• Who killed Jade Goody?
• Daisy Goodwin: This is better than real sex
• Foods that can help to prevent swine flu