YOU can expect many things with Oliver Letwin, shadow chancellor of the exchequer, but short answers are not an option. I blame his parents. Both academics — what hope did he have? “I guess,” he agrees, “the fact of them being interested in political and economic ideas did have an influence on me being interested in them, and I did spend quite a lot of time as a child and as a teenager with people who thought about these kinds of things, and talking and arguing with people who were philosophers and economists and historians probably made me more interested in arguments rather than artisan politics...”

At least, I think he said “artisan” but sometimes Letwin, 47, speaks with such flow, popping in the unexpected just to keep you on your toes, that neither tape nor human can quite make it out. Then, just when you’re floundering, he’ll let loose a high-pitched giggle and throw you a lifeline, before rushing on.

On Wednesday he will be using his very singular style of egg-headed, wordy reasonableness to provide the opposition’s response to Gordon Brown’s budget. Don’t expect him to be brief. Sitting in his cramped office in Speaker’s Court at the House of Commons, clearing a space for me at a little table on which mounds of books are piled, you get the impression that he is rather looking forward to this one.

Is he expecting any surprises? “Well, I doubt if Gordon will either change significantly the path of spending compared with the pre-budget reports, or announce huge tax rises to meet the black hole he has got. What he will do instead is fiddle around at the edges. Most of the changes we will notice 24 to 48 hours later — that’s his normal form — and there will be some sweet and cuddly things in it, and when you look through the detail you will notice that there are all sorts of other things that he hasn’t done, indexations he hasn’t performed, or, I quote, “loopholes” he has closed, trying to scrabble round and collect some pennies to try to minimise the extent of the black hole without admitting its existence and without therefore admitting Labour’s third-term tax rises...”

Right. Those “third-term” rises being the ones which Letwin and others are conjecturing that Brown, a “tax junkie”, will have to impose to pay for the increasing amount of money flowing into public services. Keep up. The poshly amiable Letwin, before he won his Dorset parliamentary seat in 1997, worked as an academic, a policy wonk and a banker for NM Rothschild, selling privatisation overseas, and it all shows.

But he’s good company.

Of medium height, slightly paunchy — he’s recently been “Atkins-ed” — with a halo of black curls framing a chubby face, he acts more like a jolly public-school teacher, shirt-sleeved and bubbly, than an earnest politician.

Behind the amiability, there’s a degree of complexity, too. Letwin boasts Russian-Jewish grandparents who settled in America, and American academic parents who settled here and then overlaid him with British Establishment values, so he’s rarely short of surprises. He is, for instance, already admired by many for his refusal to conduct political business in the time-honoured combative style. If he likes someone else’s idea, he says so.

“I just don’t think what is interesting about politics is sitting behind a brick wall and lobbing grenades at other people,” he says. He prefers “to shift the centre ground” of an argument. The jury is still out on how effective that is proving. His performance next week, tackling the first budget since he was appointed shadow chancellor in November 2003, will give a good indication.

In the longer term, Letwin has to win over the business vote before the next election. That used to be something the Conservative party could rely on. Not any more. Many in business still seem to gravitate towards Labour simply because the alternative, for the best part of a decade, has not looked that great.

But things could change. With a new leader in Michael Howard, growing resentment about the amount of red tape choking firms, and anger over the government’s U-turn on promises to encourage enterprise, there is fertile soil to be tilled. You just wonder how well Old Etonian Letwin, with his garrulous swot style, will play among the self-made men who fled the Conservative party for New Labour.

Has he won Sir Alan Sugar’s vote yet? Letwin purses his lips. “I think it would be invidious to talk about individuals,” he says, before explaining why many are starting to come round. Blame Gordon Brown. “Fifteen regulations per working day, £54 billion of additional business taxation” — managers now spend too much time worrying how to get through the tax and regulation thicket, and not enough time attending to output.

In addition, he says, Labour has pumped cash into the public sector, but to little tangible benefit. He rattles out statistics: some 600,000 extra public- sector employees, the cost of running the civil service has doubled, the NHS has received 37% more funding but only 5% more treatments, public services have got 54% more.