It was a terrible year. 1985 saw states of emergency declared in South Africa; hundreds were detained. The regime’s troops rampaged across the region. Mozambique, Botswana, Lesotho and Angola were all hit. The forces even attacked Cabinda, the Angolan enclave on the Congo River, thousands of kilometres north of South Africa’s borders.

This was the depths of the Cold War. The white regime’s troops might have crossed frontiers, but they still had the firm support of US president Reagan, pursuing what was termed “constructive engagement” with South Africa president P W Botha. Margaret Thatcher supported this position and was determined to hold the line: South Africa may follow repugnant policies, but it was not to be abandoned.

British cabinet papers released under the 30 year rule show Margaret Thatcher was well briefed on all developments. She was told that Portuguese and South African money was bolstering the Renamo rebels fighting the government in Mozambique. Her officials explained that South African forces had attacked oil installations in Cabinda owned by Gulf Oil, an American company. Their aim: “to present the operation as one carried out by UNITA,” the Angolan rebel movement that had received CIA support.

“The incident has had an immediate and serious effect not only on the Namibian (independence) negotiations but, perhaps above all, on South African/US relations at a particularly sensitive moment over the disinvestment campaign in the US.” The prime minister carefully underlined her civil servant’s words in yellow.

The line Margaret Thatcher trod was not an easy one. She was under intense pressure from former colonies like Botswana, which was having its territorial integrity violated and its citizens killed. “We have been threatened and attacked for no reason,” president Quett Masire complained to her. There was a difficult Commonwealth summit to be held in October. The prime minister knew there would be powerful calls for economic sanctions against the apartheid government, and she needed to find a response.

At the same time she had to deal with Botha - Die Groot Krokodil ("the great crocodile") as he was known - one of the most intemperate of South African leaders. It was no easy relationship. Thatcher had met Botha the year before and resisted his demands to close the ANC’s London office. Despite being one of the few friends the South African leader had, Botha was a prickly customer.

In July 1985 they had a distinctly frosty interchange. “I would have thought that it would have been the policy of the British Government to wish to combat terrorism wherever it might occur,” Botha observed, following South African attack on Botswana’s capital, Gaberone, in June. “The British Government must be aware that the ANC is controlled by the South African Communist Party and that it is therefore primarily Marxists who are responsible for violence in my country.”

Thatcher's response was immediate and tart. Two thousand soldiers and police had lost their lives in Northern Ireland, she pointed out, yet British forces had not been sent into the Irish Republic. “Rather we believe that close co-operation with the Irish authorities is the best way and indeed essential to the eventual defeat of the IRA.” Her assessment of the military operation against Botswana was scathing. She declared that Botha’s view of events “is not shared in this country. The impact on our bilateral relations has been thoroughly unfortunate and this at a time when Britain, almost alone in the international community, is attempting to resist pressure for economic measures against South Africa.” (The highlight is in the original source.)

Despite the spat, Thatcher attempted to steer what she (and Reagan) believed was a "middle course" – putting discreet pressure for reforms of apartheid, while holding out against economic sanctions.

In the run-up to the upcoming Commonwealth summit in the Bahamas, the prime minister held intensive discussions with academics and her policy advisers on South Africa. Various forms of sanctions were discussed, including attempts to halt North Sea oil going to South Africa. In the end the Commonwealth did decide to establish an Eminent Person Group to visit South Africa and the wider region to try to find a way forward.

In a “Secret and Personal” letter to Botha dated 31 October, Thatcher explained how this had come about:

The Commonwealth meeting opened with forty-five countries seeking extensive trade and economic sanctions against South Africa. In some cases this amounted to lip-service only; the interests of some countries would be severely damaged by sanctions if they were applied. But the plain fact of the matter is that nobody else in the course of the meeting was prepared to speak out against them. It was left to me.”

Thatcher went on to tell Botha that sanctions would continue to be pressed at the United Nations. “I am resolved to continue to resist that pressure, and I was encouraged to find president Reagan similarly determined when we discussed the matter in New York last week. But I need your help in this task…” (Highlight in the original source.) She then called on Botha to co-operate with the Eminent Person Group, and “explain patiently to them what your Government has done and is intending to do.”

She went on to offer her opinion that although Botha had made concessions to black South Africans his much-heralded “Rubicon” speech “did not match expectations”. She urged him to go further. “I do very strongly believe that you should be aiming to take further specific measures in the next month or so… I continue to believe, as I have said to you before, that the release of Nelson Mandela would have more impact than almost any single action you could undertake.” (Highlight in the original source.)

Thatcher’s closest supporters, Norman Tebbit and Charles Moore, claimed on Mandela's death that as prime minister she had put persistent pressure on Botha to release Mandela. Doubt was thrown on this claim when last year’s cabinet papers were released. Clearly, by 1985, it was true.