Nicholas Stadlen, who has died of mesothelioma aged 73, rose to the pinnacle of his profession as a commercial lawyer and high court judge before retiring early from the bench in 2013. Then he opened a new chapter by interviewing five survivors from among Nelson Mandela’s co-defendants and legal team at the Rivonia trial of 1963-64 for a documentary film, Life is Wonderful: Mandela’s Unsung Heroes.

Nick was in awe of Mandela and the seven people who were convicted of attempting to overthrow the state by violent means and given life sentences; and of the legal team led by the barrister and underground activist Bram Fischer, for their selflessness and courage in being prepared to sacrifice their lives for the greater good.

He wanted their values to be better known around the world but in particular among young people in South Africa, to ensure that their vision is kept alive and that the country’s hard-won democracy endures.

Nick was by chance on a visit to Cape Town in 2013 when Mandela died. He read an article about Denis Goldberg, the only white person in the group convicted alongside Mandela, and spent a day at Goldberg’s home on the mountain slopes of Hout Bay, Cape Town. “My life changed that day,” he said.

He conceived, directed, scripted and produced Life Is Wonderful, in which his interviews with many involved in the trial, including Goldberg, two other defendants, Ahmed Kathrada and Andrew Mlangeni, and two of the lawyers, George Bizos and Joel Joffe, were interspersed with archive footage.

Premiered at the Encounters international film festival in Cape Town in 2018, it won best international film. It was screened on UK TV in the same year. Nick later established a charity with the aim of having his film shown at 6,000 state schools in South Africa.

In feeding his new passion, he deployed the same techniques of forensic research and persuasion that had made him one of the leading commercial lawyers of his era. Almost a quarter of his 30 years as a barrister was spent on the BCCI case, in which he represented the Bank of England against the liquidators of the collapsed Bank of Credit and Commerce International.

Nick’s 119-day opening speech in the epic 256-day civil case still stands as the longest court address in British legal history.

One of the juniors in that case, Ben Valentin, described Nick as “one of the most persuasive, fearless and stylish advocates of his generation”, who handled mainly very large commercial disputes but was prepared to take on cases outside his specialised field.

Nick was first instructed in the BCCI case in 1991 and the litigation took place over 13 years from 1993. The case generated three appeals to the House of Lords. Nick stood up to make his opening address in July 2004. By the time he concluded in May 2005, nine months later, the BCCI case was in tatters.

The liquidators not only lost their claim for £1bn in compensation but had to pay the Bank of England’s costs of about £75m as well as their own costs. Valentin said Nick’s persuasiveness as an advocate was based on “meticulous preparation and testing of the strengths and weaknesses of his submissions” and his “masterfully forensic” cross-examination of the other side, which left no place for witnesses to hide.

Nick was born in Hampstead, north London, to Viennese parents who married in the UK after fleeing the Nazi occupation of Austria. His father, Peter Stadlen, had been a concert pianist in Austria and became a music critic in London. His mother, Hedi (nee Simon), was a political activist, philosopher and musicologist. Nick was educated at St Paul’s school, where he met his future wife, Frances Howarth, and then studied classics at Trinity College, Cambridge.

He already had a strong sense of justice. When Martin Luther King was assassinated in Memphis in 1968 Nick was on a gap year in the US. He caught a bus to the scene and then hitched a ride to the funeral in Atlanta. The experience was a formative one.

Nick was called to the bar in 1976 and practised as a barrister from Fountain Court chambers in the Temple from 1977 until 2007. He was appointed QC in 1991 and became a high court judge (and was knighted) in 2007.

Before becoming a judge, he took a break to interview a series of prominent global figures, including the former South African president FW de Klerk, Archbishop Desmond Tutu, the Israeli prime minister Shimon Peres and the Sinn Féin president Gerry Adams, for the Guardian.

Out of the courtroom Nick was warm, kind and inclusive with an immense respect for people from all walks of life. He had an extraordinary recall and was a great storyteller, but was also a good listener.

Nick was working on a film about the murdered black consciousness leader Steve Biko and writing a book on Fischer when his illness took over.

He is survived by Frances, whom he married in 1972, their sons, Matthew, William and Tommy, and grandchildren, Jude, Skye, Wilfred and Marlon, and his brother, Godfrey.

• Nicholas Stadlen, barrister, judge and film-maker, born 3 May 1950; died 5 October 2023