Michael Crick’s skewering of Theresa May on a rooftop overlooking Cape Town was highly entertaining. In the clip, Crick notes that May was active in politics in the 1970s and 1980s and asks her what she did to help release Nelson Mandela. “At that stage Mrs Thatcher believed that Nelson Mandela was a terrorist,” Crick says. “Were you a loyal Conservative party member? Did you think the same thing?”

But it was a piece of theatre based on an oft-repeated myth: that Margaret Thatcher as Prime Minister regarded Mandela as a terrorist. So how did the myth arise?

It came about during the Commonwealth summit, held in Vancouver in October 1987. Thatcher was under intense pressure from other Commonwealth leaders to support sanctions against the apartheid regime. This she rigidly refused to do.

Asked by an Indian journalist, Narinda Mohan, whether she felt isolated, she replied in characteristic fashion: “No, I do not feel isolated. I do not feel discouraged.”

She was pressed by Alan Merrydew of the Canadian broadcaster, BCTV News, who asked her: “What response do you have to a reported ANC statement that they will target British firms in South Africa?”

By this time the African National Congress was conducting a low level military operation against the South African government, although not against British investments. It was not a great success. In reality, the attacks were a form of armed propaganda aimed at getting negotiations with their leaders – including Mandela, who had already served 25 years in prison.

Thatcher replied to an earlier question by Merrydew and then picked up the issue: “I just remembered I did not answer the second part of the previous question put to me about the ANC, when the ANC says that they will target British companies. This shows what a typical terrorist organisation it is. I fought terrorism all my life and if more people fought it, and we were all more successful, we should not have it and I hope that everyone in this hall will think it is right to go on fighting terrorism. They will if they believe in democracy.”

No mention of Mandela; rather a response to an alleged threat from the ANC.

Thatcher’s long time High Commissioner to South Africa, Robin Renwick, explicitly denies the allegation in The End of Apartheid: Diary of a Revolution. He too gives the Vancouver summit as the apparent source of the claim. As Lord Renwick puts it in his introduction: “I hope that this book will lay finally to rest the contention that Margaret Thatcher was a ‘friend of apartheid’ and called Nelson Mandela a ‘terrorist’ (which, as a matter of fact, she never did).”

As I have recorded in the New Statesman, the archive reveals that Thatcher resisted the apartheid government’s requests that she crack down on the ANC in Britain, as well as deport the head of the ANC’s military wing, Joe Slovo, who was then living in London.

She also refused to supply new aircraft to the South African Airforce. Little surprise that Archbishop Trevor Huddleston, a founder of the Anti-Apartheid Movement, sent Thatcher a fulsome handwritten thank you letter, praising her stand.

As for Nelson Mandela, there is ample evidence that she believed he should be released by the South African authorities to participate in negotiations to end apartheid. Recently released papers in the National Archive show her making this point to Robert Mugabe on 1 October 1988. “We…continue to press for Mandela’s release,” she told the Zimbabwean leader.

None of this explains what Theresa May did or did not do to tackle apartheid during her time as a young member of the Conservative Party. Michael Crick was right to question her apparent inactivity on what was then such a critical international issue. But the issue of Margaret Thatcher and Mandela’s “terrorism” is a shibboleth.