Mission to South Africa seeks to provide an insider's account of the end of apartheid, based on a host of meetings which, as British ambassador to South Africa, I had with the main actors in this drama – PW Botha, FW de Klerk, Nelson Mandela, Desmond Tutu – and with many other less well-known figures who played important parts in getting rid of a fundamentally abhorrent system sooner, and with less bloodshed, than most outsiders had dared to hope.

There are plenty of heroes in this narrative, along with some cases of pure evil. Before Mandela was released from prison, he was told by Helen Suzman of the efforts the British government had been making to help secure his release. When he was released, he needed and received our help in a number of very practical ways.

Above all, he sought our support in helping to overcome problems in the negotiations with the government. He did so because he felt that, at this time, we had more influence than others with De Klerk and his mission to South African colleagues, telling me on one occasion that he regarded us as the principal supporters of the negotiating process that, however, was played out entirely between South Africans.

The portrait of Mandela that the reader will find in these pages is not the conventional hagiography. He could be dogmatic and at times distressingly partisan. Egged on by his colleagues in the ANC, he was at times unfair to De Klerk and forgetful of what he owed him. As he confessed to me, he also made a major mistake in failing for nearly a year to meet Chief Mangosuthu Buthelezi, who had refused to negotiate with the government until he was released.

Yet my admiration for him was second to no one's. Having had the chance to get to know him well before many others, I never ceased to be impressed, and at times amused, by the effect he had on normally hard-boiled visitors, who almost invariably became weak at the knees in the presence of the great man.

As Tutu observed of him, this diamond had just one flaw, which was to put his trust in colleagues who did not always deserve it. He did so not just out of loyalty, but also from political calculation.

Mandela was conscious of the fears of his ANC colleagues in Lusaka that he might start negotiating with the government on his own, and also that the township youth and half his colleagues in the leadership had more radical agendas than he did.

This led him at times to engage in rhetoric and defend positions he did not really believe in, telling me, in one very revealing encounter, that a leader who lost the support of his followers would have nothing better to do than write his memoirs.

Mandela was a far wilier politician, and could be less saintly, than some other portrayals would have us believe, though he did indeed have some saintly characteristics.

For at the time there were two Mandelas: in public, much of the time, there was the harshly aggressive, apparently unquestioning spokesperson of his party, reading out speeches written by the apparatchiks; and then there was the authentic Mandela, generous in spirit, libertarian by instinct, and inspirational to everyone he met – including me.

He used this dual personality quite deliberately to keep his supporters in line behind him. When the chips were down, as in his response to the assassination of Chris Hani, it was the real Mandela who came to the fore.

The reader will find in this account of a series of meetings with him a fundamental difference of approach between Mandela and those of his colleagues whose overriding objective was to win power and hold on to it. Much as he revered the ANC, Mandela did not believe in the supremacy of the party over the institutions of the country, including the judiciary and the press.

He told Suzman and others that he was relieved that his party did not achieve a two-thirds majority in the first democratic elections, as he wanted there to be no temptation to change the Constitution.

The least power-hungry of political leaders, he flatly refused to serve more than one term as president. At the end of every meeting I had with him, he would never fail to ask for money for the ANC, as he was programmed by his colleagues to do. I would explain to him that we provided funding for education, township projects and nongovernmental organisations, and not for any political party.

Just as he had co-opted his warder in jail and the justice minister Kobie Coetsee, who kept asking for my help in getting him released, so I found him co-opting me. I was, he kept insisting, his adviser. He also kept urging me to join the ANC. It was, he contended, a broad church, "and you think like us". This was a debatable proposition.

His next target for co-option was more ambitious. It was prime minister Margaret Thatcher. Mandela was determined, he told me, "to get her on my side", and he succeeded in doing so, though not at the expense of her admiration for De Klerk.

The reader will also find in these pages a close-up portrait of De Klerk as the supposedly conservative leader of the National Party in the Transvaal who, contrary to the expectations of many, including his brother Willem (Wimpie), set his country on an entirely new path.

He did so because, as a clear-sighted, pragmatic and principled person who took his religion seriously, he understood that the status quo could only be maintained by ever-greater violence by the state, and he had developed a visceral dislike of the paramilitary methods of Botha and Magnus Malan.

There was never any doubt that he believed in civilian control over the military, however hard he found it in practice to exert. On February?2 1990, on his way to Parliament to deliver his speech unbanning the ANC, the Pan Africanist Congress and the South African Communist Party, he told his wife that South Africa would never be the same again.

Having made the speech, he told his friends that, as an Afrikaner, he now felt able to look anyone in the eye. The absolute key as to why he launched his country on a new, uncharted course lay in the speech he made in private to the hierarchy of the South African police in January 1990.

In it, he said that the alternative to negotiations was for the state to kill thousands more people, which he was not prepared to do, and after this Armageddon, when the shooting stopped, the problem would be exactly the same as it was before it started.

Having unbanned the ANC, released a great number of prisoners and lifted the state of emergency, De Klerk found the country engulfed in a chronic state of unrest as the "comrades" in the townships flexed their muscles, the ANC continued to fight it out with Inkatha and elements of the security forces contributed to the mayhem. I never failed to be impressed by De Klerk's resolve in responding to these difficulties.

The security forces hated what he was doing. There was a real danger that some of them might actually revolt – as indeed they did in subterranean ways. His own political base was being rapidly eroded. Yet I never found him contemplating retreat, or what would have been a catastrophic desire to try to stop halfway. Meeting him in his office in Parliament or in the Union Buildings, I would find him chain-smoking behind his desk, reacting calmly to the events around him.

As I discussed Mandela's concerns with him, including at times when relations between them were badly frayed, I always found him focused on getting to the next stage and never losing sight of the goal, which was to agree a new Constitution that would give political rights to all South Africans.

As I pointed out to Mandela on two or three occasions, it is more difficult to negotiate yourself out of power than to negotiate yourself into it. De Klerk did not start off from that position.

He would have liked to see more safeguards for minority rights than ended up in the Constitution, and he believed and had hoped that a more extended introduction period of power-sharing between the ANC and the NP would have benefited South Africa.

In the midst of this difficult and turbulent process, the influential academic Hermann Giliomee, himself a reformer, but playing devil's advocate, went to see De Klerk to ask why he was making all these changes now.

"You know perfectly well that we could have held out for another 10 or 20 years," said Giliomee, causing De Klerk to get angry. "Yes, and that would entail killing a lot more people," he replied, "and what would we do then?"

De Klerk was criticised for his failure to prevent the security forces from arming Inkatha and contributing in other ways to the violence in the townships. The feeble report of the Harms commission and the connivance of the army generals allowed even the members of the so-called Civil Co-operation Bureau and the unit at Vlakplaas, under the infamous Eugene de Kock, to continue operating. De Klerk had given repeated orders to terminate all such activities, but they had become endemic in sections of the police and army under his predecessors.

It took the appointment of a much tougher judge, Richard Goldstone, and the "night of the generals" to bring things under better control. Mandela and his colleagues knew very well that, beneath the superficial deference, important sections of the police and army were flatly opposed to what De Klerk was doing. Joe Slovo and Thabo Mbeki asked me on more than one occasion about the danger of a coup, leading Slovo to suggest a period of power-sharing between the ANC and the NP.

Through the transition, De Klerk had to manage the police and army generals as best he could, and in the end succeeded in doing so. I hope that this book will lay finally to rest the contention that Thatcher was "a friend of apartheid" and called Nelson Mandela a "terrorist" (which, as a matter of fact, she never did).

Those who have continued to propagate this myth are going to have to explain away for the next several years, as the archives are progressively opened, the innumerable messages she sent to Botha and De Klerk urging the release of Nelson Mandela, the repeal of all the apartheid laws and independence for South West Africa.

She kept up these efforts unrelentingly for 10 years after becoming prime minister, applying far more pressure on the South African government on these issues than her international counterparts, earning from Mandela the accolade that, despite their differences over sanctions, he and the ANC "have much to be thankful to her for".

At the time of her funeral, De Klerk declared that "she exerted more influence on what happened in South Africa than any other political leader".

This is an edited extract from Mission to South Africa by Robin Renwick. Published by Jonathan Ball Publishers.

Renwick thanks the Foreign and Commonwealth Office for having permitted him to review his reports from South Africa, and the messages exchanged between Margaret Thatcher, PW Botha and FW de Klerk in this period, to ensure the accuracy of this account.