The key to Margaret Thatcher is simplicity. By comparison with the Big Ten of English history, alongside whom she must surely be ranked, she was a straightforward person. Whereas King Alfred, Edward I, Henry V, Elizabeth, Cromwell, Chatham, William Pitt, Gladstone, Lloyd George and Churchill were all, in varying degrees, complex and many-sided people capable of surprising, confounding and shocking close admirers, Thatcher was simple, consistent and reliable.

A touch of the numinous: From left to right, Pope Benedict XVI, Paul Johnson and Lady Thatcher in May 2009.

Her moral character as a public woman was founded on plain, uncomplicated virtues. First, and most important, came courage, the essential basis of any great political career. I never knew her to flinch or retire in confusion. But she was quite capable of a tactical retreat from an untenable position. “The great thing is to learn how to fall back under fire,” she said. I was surprised to hear her say: “I learned from Monty, never dig yourself deeper into a hole.” She was the opposite of brash. Quite without bravado, her courage was pretty calm and cool on the whole. She liked to repeat Jack Kennedy’s mot: “Don’t get mad, get even.” “The only wise thing I ever heard of him saying,” she added. She said: “I never lost my temper with Arthur Scargill, like Ted Heath did. Losing my temper is a dreadful mistake. That’s not the same as displaying anger, if necessary a terrible anger. You must be able to inspire fear. Winston used to walk up and down the Cabinet room saying of his colleagues: ‘I want them all to feel my power.’ I know how he felt though I never quite said that.”

Second came persistence. She said: “Flashes of courage are not enough. What matters is resolution. You must go on, and on, and on, and on, until people are sick and tired of your voice repeating the same thing.” When she was right, Thatcher was never afraid of being a bore, and often was. She took the view that being boring was not a political vice. The essential thing was to be right. Once you were sure you were right, you had to hammer it home, as often and as firmly, as possible. “Hammer, hammer, hammer! That’s a verb I like,” she said.

Once, early in our acquaintance, I said to her: “Government spends money on all kinds of things, often rightly. But there are only three things it must do because nobody else can. External defence. Internal order. And maintaining an honest currency. The more extra things it tries to do, the more likely it is these three essentials will be done badly.” Thatcher liked this so much she took a pencil and notebook out of her handbag and wrote it down. Years later, she suddenly began a harangue to me with: “Paul, there are three essential things a government must do . . .” I interrupted: “I told you that.” “Really? I must have repeated that maxim hundreds of times. I like it. I normally quote my father. But then I suspect I often do when I find a saying I think useful, and true. I’ll quote you in future. But then people might not take so much notice.” “Oh, thanks very much.”

Thirdly, and this was where her essential simplicity again came to the fore, Thatcher never had a long programme. Her prime objectives always sprang from her core beliefs, and these were three or four quite elementary things, such as upholding the rule of law. She fought the 1979 election on this issue, and won her two key union battles, against the miners and the printers, on enforcing the law. Once these were won, the essence of her government had taken shape. “Don’t have too many aims,” said Thatcher. “Just two or three really big obvious ones, which people know to be true, and just, and worth fighting for. Ronald Reagan held exactly the same view, I was delighted to discover. That’s why we got on so well together. And achieved such a lot, come to think of it. After all, ending the Cold War and breaking up the Soviet empire was simple, really, wasn’t it? We knew all along it was all wrong, didn’t we?”

Thatcher often used the expressions “right” and “wrong”. They came naturally to her, and she relished the terms. I never heard her use the word “evil” but I think she enjoyed other people using it, when appropriate, as in Reagan’s phrase “the evil empire”. She knew that one should never see politics in religious terms, especially in Britain. But she liked to feel that there was an unspoken religious underpinning to basic political principles. “It all ultimately came from Christianity and the Ten Commandments,” she said. “Not that one should drag Christianity in all the time — God forbid! In political life you can easily have too much humility, can’t you? And don’t be too ready to turn the other cheek. There are always plenty of Argies around to take advantage.”

Thatcher believed that there were certain things you could do, and often ought to do, but shouldn’t say you were doing them. A good example was “putting the clock back”. In a way, that is precisely what she did. She ran a government which in specific respects was reactionary, albeit with a difference. She once used to me a mysterious phrase: “I did Winston’s unfinished business.” What she meant by this was that, in 1951, when Churchill returned to power, he observed a self-denying ordinance to leave the record of the Attlee government intact, especially on the nationalised industries, the welfare state and trade unions. The decision was due partly to Churchill’s tiny majority, partly due to his own inhibitions as a former Liberal pioneer of the welfare state, but chiefly due to his conviction that Parliament had spoken and we must abide by its word.

But by the end of the 1970s, Thatcher argued, Churchill’s unspoken agreement could no longer be kept. The unions were above the law and were smashing the country to bits. They had effectively destroyed three governments — Wilson’s, Heath’s and Callaghan’s — and the public sector was grotesquely inefficient, wasteful and out of financial control. The 1979 election could be seen as a national verdict on the Attlee-Churchill consensus, and as its repudiation. Thatcher, therefore, had a real mandate for change.

Yet she used this mandate with extraordinary discretion and restraint. This is a point which must be remembered. Thatcher’s manner could be, and often was, combative. Yet her strategy was never aggressive. Her trade union reforms were quite the reverse. Unlike Heath’s, they were a step-by-step process, essentially moderate and conservative, and aimed at obvious and undeniable abuses. In the two great union battles she fought, her posture was reactive and defensive. All the attacking was done by the miners and the printers. Thatcher was never provocative. She was always ready to negotiate. She was astonishingly well-prepared, and her key advantage was that the police had learned the lessons of early, large-scale battles with militant unions — vital in the case of the miners and their flying pickets. Equally important, the new Murdoch works at Wapping had been designed with earlier experiences in mind, which proved decisive in the case of the printers’ strike. So Thatcher was lucky, as she usually was. (She is proof of how important luck is as an element in political success.) But she was also cautious, subtle, discreet, well-briefed and sensible. Indeed, if there was one characteristic which marked her tactics in her handling of the unions, it was common sense. It was pervasive and self-evident and it kept the public overwhelmingly on her side throughout. Nothing could be more historically inaccurate than to portray Thatcher as headstrong, strident and spoiling for a fight. Difficult though it may be to believe, Thatcher’s instincts and inclinations in most political situations were eirenic.

But in exceptional cases, where plain issues of right and wrong were at stake, Thatcher was an absolutist, and intransigent. The two major union battles and the Falklands invasion all fell into this category. On the last, she was taken by surprise, having been comprehensively misled by the Foreign Office and by expert lying by Costa Mendes, the Argentine Foreign Minister (as he subsequently admitted to me). Of those responsible for the deception, only Lord Carrington, himself misled, behaved with honour by resigning instantly, the last senior minister to do so on a point of principle. Made aware of the issues, Thatcher was adamant that the islands had to be retaken, if physically possible, whatever the risks. The ministry of defence was pusillanimous and deceptive, but Thatcher was saved from its cowardly advice by the insistence of the Royal Navy that a recovery operation was feasible and could be swiftly mounted. Once she was told this, Thatcher never hesitated. She remained resolute and unflinching during some very anxious moments. Thus an episode which, under different leadership — under almost any leadership at the time available — might have broken the spirit of the British people, was transformed into a triumph that kept Britain in the front rank for another generation. This was Thatcher’s biggest single contribution to the nation’s history, but it was gloriously characteristic of her ruling style at its best and deservedly set the seal on the rest of her long premiership.

Her reform of the income tax structure and the introduction of privatisation were of course far more important financially and adumbrated an economic assault on deep-rooted domestic problems which all parties had funked for 30 years. I have no doubt that the redress of local government funding which Thatcher began would have been the first step in a fundamental reform of the welfare state, which the current coalition is timidly attempting. But Thatcher uncharacteristically lost the PR battle by allowing the term “poll tax” to gain currency, and treachery in her own ranks did the rest. But it may be that no administration, whatever the quality of the leader, is capable of sustaining a policy of high-pressure change for longer than a limited period. Thatcher’s rule of over 11 years is without modern precedent in Britain.

There are many myths about Thatcher. I have dealt with her supposed bellicosity. She is also accused of being ruthless. Certainly she was loath to allow an individual to stand in the way of the national interest. But her instinct was to be generous, especially with the lonely, the humble and the weak. She grew to despise those who were good at public relations and nothing else, like Michael Heseltine, or whose sense of self-importance was exaggerated, like Geoffrey Howe. Both turned on her with venom when she — belatedly, not ruthlessly — cut them down to size.

Thatcher was never a feminist. But no one was more feminine. “I love being a woman,” she told me. “The most difficult thing for a woman in the House of Commons is to avoid stridency. I do my damndest.” Nature was good to her. She had marvellous skin and superfine hair. She had it done every day she was Prime Minister. It showed. Her clothes were superbly chosen and well looked after but she had an extraordinary talent for keeping them uncreased. Most of the time she was watched from the opposite front bench by a gloweringly jealous Barbara Castle, “It is not natural,” Castle growled. “It must be witchcraft. Sorcery!”

Thatcher was said to have no sense of humour. Not true. But it was dark, saturnine. She said to me: “As a young MP I held myself in check. I didn’t know how you laughed in a decorous, House of Commons manner. Later, I didn’t care.” You could not be married to Denis Thatcher without seeing the funny side of life. He was an encyclopaedia of jokes, many of his own peculiar invention. I used to collect them. Then, in her retirement, I would cheer her up by reminding her of Denis’s old stories: “The Charge of the Light Brigade joke.” “The ‘you could almost hear it mooing’ joke.” “The old woman in the queue joke.” And so on. She loved them. But she was like Gladstone, about whom an intimate said: “He saw jokes. But he was not often in a mood to be amused, when business had to be done.”

It was said she did not listen. Again, absolutely untrue. It depended entirely on who was doing the talking. Thatcher could not bear waffle and was astute at spotting it, instantly. Then she switched off her attention, often rudely, I fear. But the moment she caught a whiff of shrewdness she listened hard. And if it gripped her, out she would whip her pencil and notebook. These handbag implements, by the way, must still exist. The notebook will make fascinating reading.

Finally, though Thatcher was an intensely active person, she was not without a touch of the numinous. In her retirement I took her to Rome, to meet Pope Benedict XVI. As two people who loved doing business, and did it with infinite skill and dispatch, they warmed to each other. The Romans liked her too and gave her a tremendous cheer. She loved going behind the scenes at St Peter’s and placed a wreath of tiny white roses on the tomb of Pope John Paul II, whom she had revered. On our return, when I said goodbye to her at Heathrow, she remarked, oddly enough: “I enjoyed our trip into the Church. How big it is. How small we are. We must do it again.”