Oliver Letwin has written a book which will be read as long as Conservatives wonder what their party is for.

But this interview, conducted on Tuesday of this week, opens with the more immediate question of how to make a success of Brexit. And here Letwin wants “every minister” to get ready for the talks to fail.

For while “there are a lot of people still naively assuming there’ll inevitably be an agreement”, in Letwin’s view this outcome is not in the slightest bit inevitable:

“I think at the moment our best hope is that there is enough preparation put into how we deal with a situation in which there is no agreement, so that if necessary we can carry forward without an agreement, which I think may well be what happens.”

This statement is all the more striking because Letwin is known, rightly, as one of the most optimistic people in politics, who was David Cameron’s policy chief from 2005 to 2016, and at the heart of government for the last six years of that period.

In his new book, Hearts and Minds: The Battle for the Conservative Party from Thatcher to the Present, Letwin describes his failure to persuade either Boris Johnson or Michael Gove to join the Remain side in the EU referendum campaign as “one of the biggest failures of my political life”.

During this interview he adds that the “major failing” of the Remain campaign in the referendum was to resort to “hyperbole” even worse than that used by the Leavers, so making no appeal to voters in search of balanced argument.

Letwin detests “tribalism” in politics, believes strongly in the power of argument, and is candid about the way his own views have changed since the 1980s, when he worked for Margaret Thatcher. He now advocates “liberal social markets” as the cause the Conservative Party must espouse, and says the mistake of the Thatcherites, including himself, was to refuse to see the need for the “social” element in this.

ConHome: “What now has to be done to make a success of Brexit? Obviously you were very distressed by the referendum result, and as you recount in the first chapter of your book, you actually tried to persuade David Cameron to stay on. You thought he was obviously just the kind of prime minister that was wanted. But Leave won – does that also have invigorating effects?”

Letwin: “Well I certainly think that if we get things right, there will be all sorts of advantages that we can take advantage of. What we have to do is try to find ways of doing exactly that.”

ConHome: “And how difficult will it be to keep the Conservative Party together through that process?”

Letwin: “I don’t think there’ll be any difficulty in keeping the Conservative Party together. There is now a huge consensus among those who wanted to leave and those who wanted to remain alike, that we now have to carry through the exit in accordance with the democratic mandate.

“The problems now are not internal, they’re external. How do we negotiate with the EU anything which is in Britain’s interests?

“It’s all very well for us to talk about what we would like to see, or for the Prime Minister to describe what she would like to see. I don’t at the moment see any particular sign that we are in fact going to be able to negotiate that. That I think is a much bigger issue.

“I think at the moment our best hope is that there is enough preparation put into how we deal with a situation in which there is no agreement, so that if necessary we can carry forward without an agreement, which I think may well be what happens.

“If there is an agreement I think it’s much more likely to come about if the others know that we really could happily continue without an agreement. So I think from every point of view that is now the most important focus.

“And I am absolutely convinced that if there is going to be an agreement at all it will come at the very, very end. Possibly after the end.

“I suspect it’s only when we are all staring at an exit without any agreement at all that actually people will sit down with one another, and I suspect that really means Mr Macron, Mrs Merkel and Mrs May, and come to some agreement that can actually be written down on two pages about roughly speaking how to handle the whole thing, that can then be worked out over the next couple of years.

“Whether that happens or not is in the lap of the Gods. I think it’s a perfectly reasonable possibility that there will be no agreement at all.”

ConHome: “Some people say there should be a minister preparing for that.”

Letwin: “Well I think what’s needed is for every minister to prepare for that. There’s a lot of work to do yet. I think there are a lot of people still naively assuming there’ll inevitably be an agreement. I don’t think there’s anything inevitable about it at all.”

ConHome: “If there is a deal, do you think it will involve our paying a lot of money?”

Letwin: “Probably, yes…I think the idea that we’re going to end up with large financial savings is probably going to be illusory.”

ConHome: “Boris Johnson likes the idea of spending more money on the health service – a cause which appeals to the left as well as to the right.”

Letwin: “Well I think that we have to spend more money on the health service and on social care, and therefore I’ve been advocating raising taxes a bit to do that.”

ConHome: “Which taxes would you raise?”

Letwin: “That’s a good question to ask the Chancellor of the Exchequer.”

ConHome: “Do you think there has been a decline in the standard of the civil service?”

Letwin: “At its best, it’s absolutely as good as it ever was, and at its worst I doubt it’s any worse than it ever was. I think what has happened since the 1980s is that an unfortunate amount of management jargon and management speak has got into it, and this has distracted people into management rather than leadership.”

ConHome: “At an early point in your book you describe Sir Keith Joseph trying to get education vouchers through, and say, ‘officials will generally do what their political “masters” ask if these political masters are sufficiently masterly, and not otherwise’.

“Your book is going to be read by a lot of people hoping to get into government, or who perhaps already are in government, and want to learn how to get the things they want done.”

Letwin: “Yes. Almost universally the civil service will ultimately do what it is that a minister wants done if the minister is really clear about it and really determined to get it. But it does require both clarity and persistence on the part of the minister.

“And there’s no substitute for actually getting down and doing the work. If ministers are lazy that’s just as bad as being unclear, because then they don’t sufficiently understand the detail of what they’re deciding…You can’t just sort of wave your hands.”

ConHome: “You explain in the book how you were an early Eurosceptic, but then didn’t make the journey so many Conservative  MPs actually did towards deciding to leave the EU.”

Letwin: “I was amazed, as I describe in the book, by the extent to which it became clear in the referendum how many of my colleagues had progressed beyond the point that I was at in outright rejection of the EU, and indeed one of the things I try to do in the book is to chart the journey they had been on, which I hadn’t paid enough attention to in advance.

“Clearly a lot of people had come to the conclusion there never was going to be any change, and had therefore concluded they were ready to take the significant risks of leaving. It’ll be very interesting to see what view people take five or ten years from now.”

ConHome: “How grave are the risks now?”

Letwin: “I think there are significant risks. There are also significant potential advantages, and for me it was always a balanced judgment.

“I’ve more or less given up worrying about the risks at the moment, because I think what those of us in Parliament now need to do is focus on how to seize as many of the advantages as we can, and prevent as many as possible of the risks from materialising, given that we’ve had a referendum and that bit’s been decided.”

ConHome: “What were your chances, in retrospect, of persuading Boris Johnson and Michael Gove to come on the Remain side of the argument?  In the book you say, ‘I rank my failure to persuade either of them to join the Remain camp as one of the biggest failures of my political life.’”

Letwin: “Yes. I don’t know what my chances ever were.”

ConHome: “You felt you had some chances, or you wouldn’t have tried.”

Letwin: “I, er, yes, I very much hoped it would be possible to persuade Boris, yes. But whether this was a naïve hope, or whether there was some chance, I will never know.”

ConHome: “I thought one of the convincing things about him was he could see there were strong arguments on both sides. One of the very, very annoying things about the referendum was that neither side was inclined to be at all generous about the other side’s arguments.”

Letwin: “Well that in fact I think was our major failing on the Remain side of the referendum campaign. My view is that the campaign was exactly incorrectly focused.

“I think it was a campaign which in retrospect was designed to persuade those who were already persuaded to vote Remain, and failed entirely to engage with people in the middle for whom as for me this was a balanced matter.

“I think it’s very bizarre that amidst the hyperbole on both sides of the argument the hyperbole on the Remain side was if anything even more exaggerated than that on the Leave side. I don’t think the Remain side benefited at all from hyperbole and would have benefited hugely from a very calm rational  assessment of the balance of advantage and disadvantage.”

ConHome: “Why was it decided to do it this way?”

Letwin: “I don’t know. I wasn’t involved in running the campaign.”

ConHome: “Another problem with the Remain argument as presented is that it said both ‘This is a leap in the dark’ and ‘You will lose precisely this amount of money’. It couldn’t be both.”

Letwin: “Well I think that’s absolutely right, and I think the attempt on both sides of the argument to quantify it was part of the hyperbole I’m talking about. If ever there was something non-quantifiable this was it. We’re a lot closer to the actual event now and nobody has the foggiest clue [about the sums of money which will be gained or lost].”

ConHome: “Have you become a Christian Democrat? Because you’re now in favour of the social market economy, which is  what Ludwig Erhard was talking about in Germany in about 1950.”

Letwin: “Well I’m not a Christian, and not being a Christian I don’t think I can be a Christian Democrat. And I’m certainly not in favour of a politics which is closely allied to a particular denomination.

“But the social market is not the property of the Christian Democrats. It’s the common property of all liberal free-market politicians around the world who also care about social justice.”

ConHome: “Your mother, the scholar Shirley Letwin, didn’t like the word ‘social’. She said you had ‘justice’.”

Letwin: “That was one of the things she was wrong about.”

ConHome [amid laughter]: “I thought she was often right about it. Often the word ‘social’ was an evasive term, meant to convey a generalised benevolence, while detracting from whatever word it was meant to qualify.”

Letwin: “That was part of the problem. The application of the ruthless intellectual logic failed the test of emotional intelligence.

“And although I don’t, in the book, locate her as the main source of this problem, I do think she was one of many thinkers, whom I do describe in some detail in the book, and many involved in politics who failed to understand the point Keith Joseph was trying to make, about what actually needs to go alongside the free market in order for it to function in a way that makes it attractive and beneficial to society as a whole, because of its outcomes…

“Free markets don’t work for the public interest unless they’re accompanied by a certain social apparatus.

“If they are, I believe they hugely work in the public interest, which is the main argument in the book, and I think is peculiarly timely now, because I think we have to make all over again against the Corbynistas the argument that socialism does not deliver the Elysium they imagine, and that free markets are not the danger to the great bulk of the population that they represent them as being.”

ConHome: “You say in the book that politics is mostly an argument about means, not ends. One of Corbyn’s ends would be much greater equality, I suppose.”

Letwin: “Yes I actually think – obviously at the extremes, if you’re talking about a bloodthirsty tyrant either of the right or the left, then the ends may be entirely malign, and wholly different from those of democratic politicians of any reasonable kind. But once you’re inside the framework of the kinds of debate that go on between politicians in Britain, I think on the whole everyone pretty much agrees what it would be nice to see.”

ConHome: “Would you like to say a word or two about what the ends are?”

Letwin: “I think pretty much everyone would like to see a society that’s very free, but at the same time where nobody is in distress, and where income results are sufficiently equally distributed so that society can be both homogeneous and prosperous enough for real equality of opportunity for children and real security in retirement for the old, and a society that’s broadly capable of sustaining both our heritage and the beauty of our natural environment, and a culture which goes therefore beyond just economics – I think these are things which people very often say are motherhood and apple pie, because actually everybody shares them.

“It’s quite important to remind ourselves of the motherhood and apple pie that we do actually share, rather than thinking we disagree about everything because we disagree about how to get to these things.”

ConHome: “A few High Tories might feel a twinge or two of pain at your account.”

Letwin: “I don’t think there are very many people on any part of the political spectrum who would actually disagree with very much that I’m describing as the desirable end. I really think it is almost entirely a discussion about means.

“Which is one of the reasons why I’ve always thought politics ought not to be conducted as a ya boo sucks matter. Opponents are not evil, they’re either right or wrong about this or that.”

ConHome: “So obviously that applies to Corbyn and co as well – they’re not evil.”

Letwin: “Exactly. I’m absolutely persuaded that he’s an entirely well-intentioned man. He’s completely wrong about the results of his own policies, which I think will lead, as they have done in Venezuela, to the exact opposite of what he hopes.”

ConHome: “I remember years ago you were on friendly terms with both Diane Abbott and Glenda Jackson [who defeated Letwin in the general elections of 1987 and 1992].”

Letwin: “I’ve always tried never to mix politics with personal antagonism. Why turn the whole thing into a personal vendetta? It doesn’t need to be. It’s a lot pleasanter if it isn’t but also it’s a lot more productive.

“One of the things which I try to bring out in the book is that if you don’t conduct it that way, you can actually recognise the amazing fact that we do agree about a lot of things.”

ConHome: “Well you mustn’t completely undermine the adversarial basis of our politics.”

Letwin: “I don’t really see much joy in the adversarial basis of politics. There’s quite enough people genuinely disagree about not to need to disagree about tribal questions. Tribalism in my view is an unmitigated mistake.”

ConHome: “I think it helps provide the necessary sense of community.”

Letwin: “I don’t. I think the community exclusively should be the country, the county, the village, one’s friends, not the parties. I think on the whole in the 18th century they were right to regard factionalism as a very dangerous thing.

“You’ve got, unfortunately, in a modern highly sophisticated representative democracy to have parties, but it’s incredibly important not to have tribes.”

ConHome: “David Hume was very against faction, wasn’t he?”

Letwin: “Yes, and he was right.”

ConHome: “You’re a rather radical figure, aren’t you. Why did you join the Conservatives?”

Letwin: “All parties are coalitions. You choose a party that more overlaps with your own views than any other.”