In a forthcoming book on Conservative education secretaries since 1979, he reveals: “The first time that I met my civil servants . . . I said to them: ‘Do you realise that the education system of this country is failing 50 per cent of the population; 50 per cent of the children go through the schools and it does them no good at all?’ “The civil servants all looked at each other and one said very smoothly: ‘Oh, Secretary of State, please don’t say 50 per cent; it sounds an awful lot. Let’s say 40 per cent.’” Thus are catch-phrases born. But the concern of the late Lord Joseph (as he became) with those at the bottom of the educational pile, and his determination to move the education debate from structure to standards, emerge as essential preparation for the reforms of the late 1980s and early 1990s. For it was not until the third Conservative parliament, with the principal trade union and economic reforms in place, that the Thatcher government really turned its attention to social policy. The tortured intellect of Sir Keith, who was Education Secretary from 1981 to 1986, looms over this collection of “conversations” with the other five men and one woman who have occupied the post in the past 18 years. All pay tribute to his influence, while regretting that so much of his tenure was dominated by a vicious pay dispute with teachers that he was unable to resolve. Not all are as brutally frank as Mark Carlisle (1979-81), who comments: ”... I am not quite sure what he actually did”.

By the time the book’s editors, Peter Ribbins and Brian Sherratt, came to conduct their interviews, Lord Joseph was too ill to meet them. The “conversation” with him is pieced together by Clyde Chitty from earlier interviews with him and Stephen Ball and from published statements. The others were conducted by the editors in a style closer to that of Cliff Michelmore than Jeremy Paxman.

The characters shine through. There is the old-fashioned charm and decency of Mark Carlisle, who still worries about the children left behind in the race for popular or selective schools, bless him. While he says his achievements have been underrated - he reversed comprehensive compulsion, extended parents’ rights to choose a school, introduced the Assisted Places Scheme - he rates the 1981 “Warnock” Act on special education only third in importance among his measures and seems pretty vague about its contents.

No vagueness about Kenneth Baker (1986-89), whose verve and bumptiousness come bouncing off the page. From his resolution of the teachers’ pay dispute which gave him, he says, the position “to establish educational policy for the following nine years ...or even for the rest of the century and beyond”, to his invention of the grant-maintained school and the driving through of “the biggest single measure of social reform which was undertaken in the Thatcher years” (the 1988 Education Reform Act), it was one long success.

His biggest regret? That he didn’t push harder for an extra teaching period a day. That would, he says, have solved many of the problems that arose with fitting in the national curriculum. But, having just resolved a pay dispute with an agreement on hours, it wasn’t possible to reopen the issue. What about “the setting up of a complicated system of assessment”, asks Peter Ribbins gently. “That was a regret,” concedes Baker. He would have liked to stay longer to sort it out.

It was probably a regret to John McGregor too, the nice and competent man who took over in 1989 with the task of picking up the pieces so jauntily scattered by his predecessor. Indeed, much of the tenure of Kenneth Baker’s three short-lived successors - John McGregor (1989-90), the combative Kenneth Clarke (1990-92) and the unfortunate John Patten (1992-94) - was spent picking up those pieces and trying to create a manageable national curriculum and associated testing system. “The devil is in the detail,” as Clarke says.

There are wonderful vignettes, such as Mr Clarke’s visit to a Surrey primary school, where the head gave him a ferocious lecture about the appalling effect on her school of the first round of tests for seven-year-olds. She won herself a seat on the School Examinations and Assessment Council and, by the sound of it, did more to guarantee a rethink than anybody else.

Ribbins and Sherratt ask about the apparent paradox in Conservative education policy: the centralising or “strong state” tendency behind the curriculum reforms against the decentralising or “free economy” tendency of more parental choice and greater autonomy for schools. The question causes few problems to these present and former Education Secretaries - as Gillian Shephard replies: ”... if you’re going to have a diversity of provision, then you must have certainty about what is being provided.”

All express regrets, not so much about mistakes as about having their time at education cut short - perhaps surprisingly, in the light of their common view that life as an Education Secretary is unremittingly hard work. John McGregor, whose tenure was ended when Sir Geofrey Howe resigned and he was moved to be Leader of the House of Commons, says simply: “It is the one major regret of my political life that I did not stay at education longer.”

All political life is said to end in failure. It also ends in eating your words. There is John Patten, who said he would eat his “academic hat garnished” if by the time of this next election more than half of England’s secondary schools were not grant-maintained.

And there is Mrs Shephard who, interviewed shortly after becoming Education Secretary, said: “I’ve got a great variety of things that I want to do but legislation, I think, is out.” That was before John Major started putting on the squeeze. Mrs Shephard’s 70-plus-clause Education Bill, the second in her period of office, had its turbulent third reading in the Commons this week.

Radical Educational Policies and Conservative Secretaries of State by Peter Ribbins and Brian Sherratt will be published by Cassell on March 20.