Moore replied: “I’d say these things called jokes, which have punch lines and a set-up and say things like ‘there’s an Englishman, an Irishman and a Scotsman’, are fundamentally male, and she had absolutely no understanding of them whatever. But this does not mean she had no sense of humour. It’s just different. She had a sense of wit because she had a verbal directness which is almost biblical Judaic. Something would come out quickly in riposte, which was sort of funny, yes it was funny really, it was crisp. Another thing was a sense of fun which was about enjoying a situation. There’s a sense of theatre. So one reason why she was such fun to work for I think – not fun to work with, as a Cabinet member, but to work for as say private secretary - is that she’s always terrifically enjoying all this, and there’s a pantomime element in her which is camping herself up, spoofing herself, you know, wanting to go and tap you on the shoulder and wave the handbag. You know, playful. She didn’t understand double entendres at all, of course. That comes into the book.”

The first volume of Moore’s life of her, Not For Turning, is published today, and contains (among much else) so many comic moments that I did not wish this aspect of her to get forgotten because we had plunged into more momentous themes.

How does Moore think Thatcher should be remembered by the Tories? This question was inspired by Lord Lexden’s recent piece for ConservativeHome about the Primrose League, the amazingly successful organisation set up in memory of Disraeli. What is going to be done on 8 April 2014, the first anniversary of Thatcher’s death?

“Well I think this is a very important question,” Moore said, “and you do need to strike while the iron is fairly hot, but on the other hand you must get it right. So I haven’t got an answer. I think they need to think about it quickly, but coolly. My instinct is that what it should be used for in Tory party terms is grassroots revival, rather than ideology or scholarships. Specifically what’s happened within the Tory party is the collapse of the tribe and her early life shows the incredible strength of the tribe in her era, and she’s the only one really to rise through the tribe. Even Heath didn’t quite do that because though he was an outsider, he became the darling of the insiders.”

In Moore’s book, he points out that when Thatcher was selected in January 1949, at the age of 23, as the Conservative parliamentary candidate for Dartford, a seat held by Labour with a majority of 20,000, the local Tory party had 2,300 members, a number which by 1951 she had increased to the still more extraordinary figure of 3,160.

“The Tory party will never be like that again,” Moore continued, “but there must be a modern way of doing that with social media. It should come from the grassroots. Actually the obvious thing for it to come from is ConservativeHome.”

“Did she have a favourite flower?” I inquired – the primrose from which the Primrose League took its name having been alleged, almost certainly wrongly, to be Disraeli’s.

“She was very fond of flowers,” Moore replied. “She actually liked the rose very much. That was probably her favourite flower, but Labour pinched that one. Can you have a blue rose? No. There’s a touching thing, right at the height of the Falklands, when she really has got a lot to think about, when she hears that the wife of Sir Antony Acland, the new head of the Foreign Office, is very ill, and she picks a lot of roses at Chequers and they come with a handwritten note offering ‘the scent of flowers from an English country garden for you’.”

I suggested to Moore that neither her admirers nor her detractors generally understood what a mixture of opposites Thatcher was: very brave yet also very cautious; a serious woman who was attracted to raffish men; a puritan who was tolerant of others’ weaknesses; a conventional Tory, yet the first woman Prime Minister.

“This is true,” Moore said, “and it does go right through, and it’s part of the key to her success, I think, because she’s always wrong-footing people. So for example, because she’s very formidable and combative and successful, that led a lot of people to say ‘well she’s really a man’, because they think only a man could be formidable and combative and successful. It’s of course complete and utter rubbish that she’s really a man. You could not be less like a man, it seems to me.

“And similarly because she had a reputation for being serious, which was true, people were surprised – this is a very important point in her public campaigning I think – they were surprised by how she was electric, not boring. People think serious means boring, and there were occasions sometimes when she could be boring, more in private actually, sometimes in conversation she would wish to say the same thing again and again about the serious defence of liberty or something.

“But in public there was tremendous zest. She absolutely loved it when she charged into the supermarket and hurled the goods into the trolley, or the famous cuddling of the calf in the ’79 election, or getting on the prow of a ship, or flying up in a helicopter or something. She’s as people now say ‘up for it’. This surprised people and also it gives you integrity, because something which I think was very clever about Mrs Thatcher and campaigning and media was how she was basically authentic AND artificial.

“She was intensely artificial: the hair, the make-up, the clothes, the voice, everything very professionally worked out to achieve certain effects. But the reason they achieved those effects was they all reflected a reality about her. Artificial yes, bogus no.

“She was extremely disciplined. She knew that everything she said or did, where she went, whom she visited and so on was part of the overall message. So she was the first really successful self-spinning and spun politician in British politics.”

Moore spoke with intensity about how disciplined she had been in getting her message across, and contrasted it with what happens now: “It just drives me crazy, what’s so bad about current politics is that on the one hand there’s the people no one can believe, playing this game of spin, and on the other hand there’s all these posturing, rather undisciplined Tory backbenchers who just say, ‘I just speak my mind’. You mustn’t say what you don’t believe but you mustn’t say everything that comes into your head. You have to think of the good of the whole.”

We reached her battle against Edward Heath. Moore is the first historian to be given access to the diary of Airey Neave, who became Thatcher’s campaign manager when she stood against Heath after he lost the two general elections of 1974: “It seems to me to be an absolute treasure trove, and it shows various things which are very interesting. One is that the one strong view coming through from the back benches was that Heath ought to go. There was no other agreement really. So they’re casting around. Neave was casting around and right up to the eleventh hour he can’t decide.

“The point about Airey was that he reached the parts she couldn’t reach, because she never knew how to intrigue within the Tory party, indeed it would have been disastrous if she had. What she had to do was to reach the knights of the shires. Contrary to the jokey Julian Critchley stuff about the garagistes, the peasants’ revolt, it wasn’t that, most of these people she had to win over were petty gentry, professionals, people who’d been in the war, and their attitude was a regimental attitude.

“They were like the officers in the field who were fed up with the generals at HQ. And they weren’t revolutionaries at all, but they thought they were poorly led and it was all going wrong and the country was going down the drain. And they on the whole didn’t think of Mrs Thatcher as left-wing or right-wing, they just thought she was a brave girl, we’re fed up with Ted, let’s give her a go, and you needed someone like Airey to deal with all that. And again the advantage goes to the brave. A lot of people who wanted to succeed Heath, of whom Willie Whitelaw was the most senior, Jim Prior would be another, Geoffrey Howe, didn’t challenge, so she was the only person in the first round to challenge apart from Hugh Fraser, who was a delightful man but not important politically.”

Thatcher benefited enormously in the leadership election from being underestimated: the other candidates imagined she would clear the way for them, but instead established an unassailable lead. Heath hated being challenged by a woman, and in the book Moore quotes him saying of Thatcher to a close friend: “It’s a matter of opinion, whether you think she’s a woman or not.” As Moore observed during our interview, “Poor old Heath didn’t understand Thatcher at all.”

Because Moore takes Thatcher seriously, he is also able to observe how she quite often got her own way by using methods which she herself denied using:

“Another important point to come out of all this is how astute she was. She always denied this. She would never admit to cunning calculation. The Margaret Thatcher story of being a conviction politician is true, but is also misleading, because she was brilliant early on, less brilliant later, at conciliating who she had to conciliate, and at having a good doctrine of ripe time and unripe time, and she was never deflected from her main purpose, but she often took diversions or slowed down the pace.

“In the crisis of 1981, there’s a superb memo which no one’s ever seen in full where John Hoskyns, the head of her Policy Unit, absolutely rips into her about her leadership style. It’s a rather brilliant encapsulation of her bad qualities.”

But Hoskyns’ solution – that she should have a strategy – was in Moore’s view misguided, for no strategy could reflect “the extraordinary shifting reality of politics”, in which it is best to have an aim, or aims, and do whatever is necessary to advance them.

Moore recognizes that there is a case to be made against Thatcher: “Well I think there are bad reasons for being against Mrs Thatcher, and there are respectable ones. The bad reason is an atavistic reaction against her sex, class and manner, by the type of person who regards politics as a game of disapproval, in which if you don’t agree with somebody you think they’re morally inferior to you. And all that’s very bad and unattractive and shows up all sorts of snobberies and priggishness. On the other hand, Mrs Thatcher stood very strongly for a series of beliefs which a lot of people very conscientiously disagree with, and she would perfectly have understood and welcomed their enmity. It’s rather like what she used to say about the Labour party. She was fiercely antagonistic to the Labour party, but she always said one strongly nice thing about them. If she liked a Labour politician she said, ‘Well he stands up for the underdog.’ And that what she thought was good about the Labour party, and there were many Labour MPs who she liked, Eric Heffer, for example, Charlie Pannell, Jim Griffiths, Frank Field, several others.”

In his account of the Falklands, with which the book ends, Moore says he stresses “her personal emotional turmoil – so much is at stake, her reputation, her career, the awful, awful feeling that she’s sending people to their death, the loneliness of it, the incapacity to do anything a lot of the time, because you know sometimes it’s completely out of the hand of politicians, it’s in the hands of soldiers and sailors and of the sea, the weather. That’s very, very vivid and very important, and that’s why she lived more intensely then than at any other time. The other thing is that contrary to her own account, the pragmatism, because she glosses over a great deal. One of the terrible moments is when Francis Pym came back from America with a new plan which she described as ‘conditional surrender’ and the sacrifice of Britain’s honour. And I think she was right to resist his plan. But later on in the Falklands campaign she gave very similar concessions, because it’s all part of a game about working out what you need to do to be seen to be satisfying world opinion and to get America to do what you want. She made concessions that were fundamentally very similar to those that Pym brought back from the United States, and of course because she made them she didn’t see them in the same light. I think she’s very frightened of a rival coming back from America and presenting something as peace in our time. So not only does she think it is wrong but she thinks this bastard is trying to take my job. He’s a threat. She’s very, very concerned always with political threats by her colleagues. But she as I say did something not dissimilar not long after it. It’s terribly complicated in the detail so you must read that bit. In order to keep the Americans on side she basically does agree to something that would have conceded British sovereignty over the Falklands. And it’s very important to get this right. She was making a calculation which turned out to be completely correct that the Argentines wouldn’t accept it and she would not be taken at her word. So the idea that she’s really trying to give away the Falklands is rubbish. But it’s a huge risk and it shows what a tight corner she was in.”

The Falklands War blew up with amazing speed. “But I think the Falklands does show her at her best, because it shows this combination of fantastic courage and verve with such careful study and sense, absolute concentration on how to get it right, including the bit she will never admit, the twisting and turning. She’s a great twister and turner. Always with the aim in view, but nevertheless, she didn’t survive by being a conviction politician alone, she survived by being a conviction politician and a twister and turner.”

The two most famous British politicians Moore was unable to interview were Keith Joseph, who was dead, and Willie Whitelaw, who was not well enough. Nor was Ronald Reagan in a fit state to be interviewed, though Nancy Reagan was, as were Denis Thatcher and Muriel Cullen, Margaret Thatcher’s elder sibling, who supplied a large and hitherto unknown collection of letters written by the young Margaret. “Obviously it’s absolutely fascinating to have the letters of someone when they’re not famous, so they’re quite unselfconscious. It’s such an absolutely classic English story of the middle classes and Methodism and provincial life, given a huge drama by what happened afterwards.” Moore has also discovered, in Tony Bray, a hitherto unknown boyfriend.

To Moore, one of Thatcher’s great strengths was that she did not examine her own life in the way that a man might have done: “That’s the trouble with male vanity. You say, ‘Ha, ha, I fooled that bastard.’ She was fooling people a lot, but she would never have said that, even in private. “

After the interview had ended, I turned to the book. The first part does indeed read like a novel of provincial life: something by Philip Larkin or Muriel Spark. It is full of arresting detail. When Margaret Roberts, as she then was, arrived at Oxford, she was one of only five women chemists in the university. At the age of 25 she took her driving test: “Because she had two outstanding driving lessons paid for at the moment of passing, she went ahead and had them all the same, showing a virtually inhuman determination to get value for money.” In her whole life, she never entered a pub by herself. Despite Thatcher’s intensely private nature, it seems to me that Moore has given us a more humane and rounded portrait of her than has ever been seen before.

Margaret Thatcher, The Authorized Biography – Volume One: Not for Turning, by Charles Moore, is published by Allen Lane (£30).

Young Margaret, Life, Love and Letters, a documentary based on the biography, will be broadcast next Saturday at 9.00pm on BBC2. It contains the only interview ever filmed with her first boyfriend, Tony Bray.