Oliver Letwin's office rings to say he is running five minutes late. The call seems courteous but unnecessary, unless the shadow home secretary fears that any minor delay on his part may signify a relapse into his Macavity habit. During the general election campaign, Letwin inadvertently disclosed that the Tories were aiming for a £20bn tax cut by 2006. Then he disappeared. Though the groomed farmland of his West Dorset constituency is not to be compared with an Afghan cavescape, the bunker-busters of the media failed to dislodge Letwin. Posters went up. Rewards were offered. The episode was, as he now acknowledges, a terrible blunder.
"I learnt two lessons from it. The first is that I shall never again in my life give any interview unless it's on the record. And second, if anything I say ever again becomes the subject of huge interest, I shall be in front of the television cameras in 30 milliseconds." So why did he agree to hide? Was it, perhaps, because William Hague ordered him to? "I am going to maintain a total silence," he says virtuously.
On the substantive point of a £20bn cut (as opposed to Hague's £8bn target), Letwin seems astonishingly unrepentant. When I ask why he mooted such an alarming plan, he says: "Oh. Well, I am surprised that you find it alarming. I think the mistake we made was to think that the people of Britain realised the truth: namely, that a very small part of our national budget is spent on things people notice - schools, hospitals and so on.
"We now understand that no one will pay the slightest attention to claims that it is possible to control the growth of public spending over time - which is all we wanted to do - unless we can first convince them that we can provide better schools and hospitals and roads and trains without spending a colossal extra amount of taxpayers' money. That is where we are now focused."
So the £20bn is still there for the saving, once state shrinkage can be sold to the public? Although Letwin allows that the old figure will have to be updated ("the £20bn is out of the window in the sense that it was [calculated] about a year ago"), he believes that the sum was never a mad aspiration, simply an idea rolled out before its time. "Oh, yes. Controlling the growth of public spending is of course a Conservative aspiration."
While bullish about Tory credo, Letwin is unexpectedly diffident over whether the current line-up will recover fast enough to implement it. "Last time round, we hadn't learnt how to engage in opposition. We were too inclined to spot an opportunity for grabbing a headline and too exposed to having to reverse the decision later." This Hagueish nemesis is presented by Letwin, who served in his shadow cabinet, as collective culpability. "All my political formation has been in government . . . I'm as guilty as anyone of not understanding what it is to be in opposition. What I hadn't come to terms with, but have now, is that what people most want to know about an opposition - as and when they're not inclined to vote for the government - is this: are these people in whom you would safely put your trust to let them run your country?"
Surely the voting public is some way from this epiphany? "Nowhere near, nowhere near," he cries. "We have a huge hill to climb. It is very large. We have to re-establish, with millions of citizens who are very disenchanted and very cross, our credibility as an alternative government. That isn't something we are going to be able to do in a week or a month or a year. It's something we have to try and do over four years."
Is even that achievable? "I don't know, I don't know. I hope it's doable . . . We have been in a long and bleak abeyance, and I want to see us come out of that. We have to realise that, in order to do so, we have to behave in the ways I have described (no more yah-boo politics) and realise that it's going to take time. And we have to recognise that it's difficult. The outcome is not preordained."
So it might take two terms? It might take another change of leadership? I expect a flat denial of both, but he says: "None of us, standing here today, is in a position to know how far or how fast we will advance. What we are in a position to know, I hope, is what kind of self-restraint, self-discipline and leadership of purpose we need to engage in order to get there."
For a senior member of the shadow cabinet to express doubts about whether the Tories can win the next election is unusual. For Letwin not to contradict the suggestion that Iain Duncan Smith may not be the anointed saviour seems even more bizarre. Does he aspire to lead his party? "No. I am an unusual politician in that respect. I am ambitious, and I would love to hold one of the major offices of state. I understand the quantum leap in demands.
"I find my present role on the verge of being too demanding. I actually like my family life. I like my social life. I have twins [Laura and Jeremy] of eight, and a wife [Isabel, a lawyer] whom I rather like. This is the limit of what I want to do. I don't want that kind of demand. I'm astonished there are people who do. I know from talking to Iain that he had to think very hard before taking it on. Just after William resigned, I had conversations with Iain which made it perfectly clear he was hesitating. I know he wavered for some days . . . I think I'm a useful commodity, because I have no ambitions in that direction."
Which is what they all say. Still, I believe him. In assorted ways, he is an unlikely politician. His mother, Shirley, an American political philosopher and a leading thinker of the Thatcher revolution, thought politics "a pretty lowbrow activity. She would have been very distressed to discover I was occupying myself with practical matters. Part of the reason is that I'm not quite up to the job of doing abstract things, as I discovered rather painfully." How? "Oh, by discovering that if I took all my philosophical works and burnt them, it wouldn't make much difference to anybody," he says with a high, hurt laugh.
While the sales of his books on ethics, economics and education do not imply Harry Potter for pointy-heads, he can well afford a disarming streak of self-deprecation. After Eton and Trinity College, Cambridge, his first job, before a spell as a merchant banker, was a stint with his mother's friend Keith Joseph. He emerged inspired.
"I always feel Keith is looking over my shoulder. I often try and judge how I should behave by thinking of what he would do. He took positions he thought were right. It didn't always make him popular, and some people thought he was mad." Probably because of the way he used to sing in railway carriages, I say, but Letwin looks politely askance. "I don't deny he was eccentric. But I mean people thought he was mad not to take opportunities of low cunning."
While no one would accuse Letwin of the rat-like wiles eschewed by his guru, he seems a sinuous pragmatist. "Of course, otherwise I wouldn't be in politics." He started out as a Michael Portillo man, switching later to Duncan Smith, whose line on Europe meshed with Letwin's. ("I would never vote for entry into the euro.") Otherwise he remains hard to pigeonhole; billed variously as a right-winger and a born-again liberal.
He had not expected the home affairs portfolio, and he does seem slightly adrift or oppressed. When, at the start, I offer congratulations, he says: "Commiserations, probably. This job is difficult at the best of times, and in the current climate significantly more so."
In the main, he agrees with David Blunkett's anti-terrorist legislation, although he opposed the religious hatred clause and internment, preferring to enable a home secretary to send suspected terrorists back to their own countries to face retribution, the death penalty included. Assassination seems his favoured option for Osama Bin Laden. "There are times when an enemy is so dangerous that you are entitled to kill him."
In parts of his brief, he seems oddly lacklustre. On prisons and young offenders' institutions, there is "an enormous amount of work to do". On judges, he is less confrontational than Blunkett and thinks that a beefed-up parliament (but how?) would smooth the fracture lines between the executive and judiciary. If he lacks Blunkett's cauldron of ideas, he does exude a reasonableness lacking in some colleagues.
Naturally, he resents the idea that the shadow team is, pace Charles Clarke, a convocation of nutters. "Of course it isn't. Not that you'd expect me to say so if it was. But it isn't." Look at Michael Howard ("extraordinarily capable"), Michael Ancram ("solid and reassuring"), David Willetts ("one of the most moderate intellectuals"), Liam Fox ("absolutely sane"). "I could go on," Letwin says. (He does, a bit, but still we do not get to Bill Cash.) Anyway, the real problem may be that the Tories aren't half crazy enough. A yearning, endorsed by Letwin, for consensus politics, coupled with the unanimity of the war effort, has rendered the opposition uniquely effete. As he acknowledges: "There are penalties, and we accept them. People do not pay us very much attention."
It is 8.30pm when I leave the Commons. "Are you going home?" he asks, in a tone of such wistful envy that I wonder if he has the stamina for the long slog. He will be in his office until 2am, he says. Once so elusive, the Scarlet Pimpernel of politics must now be omnipresent. I'm not sure that Oliver Letwin has decided which fate is worse.