Drawing on his second cigarette within half an hour, Andrew Mlangeni recalls how tobacco was hard to obtain in prison on Robben Island.

“I try to stop, but it takes willpower,” the 93-year-old said.

Speaking among the celebrations for Nelson Mandela’s 100th birthday in the quiet residence of the South African ambassador in Kensington, the one quality Mlangeni does not lack is willpower. Convicted of high treason in the notorious Rivonia Trial in 1964 along with Mandela, he spent 26 years and four months in jail from 1963 to 1989. One of the first to be sent to China by the ANC for military training in 1963, and arrested soon after his return, he is now, alongside Denis Goldberg, one of the two surviving members of the trial.

He wears his bravery lightly, and is seen as one of the quietest of the 11 that were put on trial for treason. Sabotage was a capital offence, but he agreed with Mandela that their sole aim at the trial had to be to put apartheid in the dock, whatever the cost to themselves. If sentenced to death the group would not appeal. Their courage was all the more remarkable considering that from the outset their lawyers warned them to expect the worst, and not the life sentences eight of the defendants were given. (Two were acquitted and charges were withdrawn against a third.)

Mlangeni is in London to help launch the Robert Kennedy human rights centre and an exhibition opened by Meghan Markle on Mandela’s life. He walks with a cane and apologises for his voice, weakened by a plane journey from South Africa.

“I never wished to be in prison even for one day, but I did it for freedom,” he says. He likens the loss of freedom to the loss of a body part : “It’s like you have five fingers. You do not realise the importance of each finger. It is only when you are missing one for whatever reason that you realise what you have lost. When you are in prison you realise what is outside – friends, family, children, wife, you name it. But when you are outside, you do not appreciate your own children and your own wife.”

He adds: “I was sent to jail when my youngest child was aged seven. I only started to see my girls when they were already married with children. I could not attend their weddings or the funeral of my twin sister. I felt when she died I had lost part of myself.

“We were categorised as prisoner D – worse than rapists – so you receive visit from your wife or children only once in six months, and write a letter once every six months.”

Did he detest his jailers? “I did not hate my jailers. We thought we should educate them. By hating them, you do not benefit anyone or anything. They were told they were going to be dealing with murderers, rapists and dangerous people, but when they met us they realised we were educated people – some are doctors, lawyers, polished people. Some of them even started to study like us.”

The former political prisoner demonstrates how he used to sleep during the launch of the replica of Nelson Mandela’s Robben Island prison cell in 2016. Photograph: Nic Bothma/EPA

As the apartheid regime started to crumble, and the government of De Klerk looked for a way out, Mlangeni remained rigid.

He recalls that Mandela was approached by Kobie Coetsee, the justice minister, who, in 1982, told him he would remain in jail but at least three of his comrades in Pollsmoor prison – Walter Sisulu, Raymond Mhlaba and Mlangeni – would be released.

Mandela, Mlangeni recalls, “thought if we were released it will strengthen the forces that are outside fighting for freedom. It would strengthen the call for all political exiles to be allowed to return.”

But, he says, “Mandela said he had to consult us, and we rejected the idea. We said: ‘We are all in one case at the Rivonia trial. We were all sentenced to life imprisonment. What will the people say if we are seen outside and yet one of us remains behind. Go tell the government, we will not accept the offer.’ Mandela was terribly disappointed.”

Mlangeni remains a stalwart of the ANC, and was, until a few weeks ago, chairman of the ANC’s Integrity Commission, a position from which he repeatedly urged former President Jacob Zuma to resign due to the overwhelming corruption charges.

Zuma resigned in February and now faces a corruption trial. Mlangeni criticises him for his links to the Gupta family, the Indian-born family accused of using their wealth to capture the South African state. Mlangeni spent 10 years in jail with Zuma, but says the former leader fell among bad people, and, if necessary, must go back to jail.

“The Guptas did everything for him, even choosing his cabinet ministers. Zuma sold his power to the Guptas,” he says.

The ANC legend says it is a good question why more was not done earlier to expose the corruption and bring it to an end. “It is said the government was soft on Zuma. The opposition parties are saying ‘we cannot excuse you. You are part of the group that did not question or did not say a word to condemn Zuma.’”

He suggests a systematic failure: “No one wants to lose their jobs, they get money, they get good salaries, they get the facilities that go with being ministers, you get free housing, security transport. So once you are a minister and start to oppose the person that appointed you, then within 24 hours, you lose your job. People did not want to lose their job so they kept quiet.”

He is, by contrast, full of warmth for his friend Cyril Ramaphosa, Zuma’s successor, and in particular his use, in his state of the nation address, of the Hugh Masekela song Thuma Mina (Send Me), in essence a call to lend a hand and volunteer in a collective effort to turn the nation around.

Faced by factional forces inside the ANC, and huge economic challenges, Ramaphosa will need all the support he can from one of the last great original, unsullied figures of the liberation struggle.