It was a yellow scrap of paper found in his mother’s Belsize Park flat that set human rights barrister Philippe Sands on a quest that led to personal and professional discoveries.

In an old briefcase of papers was the Norwich address for a Miss Tilney, who it emerged had saved his mother’s life, transporting her from Nazi-occupied Vienna to Paris in 1939.

“I found the story behind the extraordinary woman who saved my mother’s life. Elsie Tilney was an evangelical Christian missionary who met my grandfather Leon in Paris.

“He asked her to bring his infant daughter from Vienna. They never saw each other again and she was interned in Stalag 121 in Vittel.”

Answering questions about Leon’s origins, how and why he was separated from Ruth and wife Rita, the Hampstead resident uncovered connections with three other men, one a defendant at the post-war Nuremburg Trials, the other two prosecutors.

Standing trial was Hitler’s former personal laywer and Governor General of Poland, Hans Frank. Prosecuting were two lawyers charged with creating legal definitions for acts so gross they almost defied langauge.

Hersch Lauterpacht and Rafael Lemkin invented the terms ‘Crimes Against Humanity’ and ‘Genocide’ that Sands still uses in the criminal court in the Hague where he has been “actively involved in cases of mass killings in the last 20 years, in Yugoslavia, Congo, Iraq and Rwanda”.

“In the 1940s there were no international rules that stopped a state from killing its own population. Those men came up with two new concepts; crimes against humanity; the protection of individuals, and genocide; the protection of groups. There’s a tension between them that goes to the heart of who we are. Are you an individual or a member of a group? How should the law protect against mass killing? I oscillate between an empathy for how in law you have to focus on the individual, but in life you are members of a group.

“We grapple with torn loyalties, try to reconcile essential questions of identity and decision making.”

During the trials, Lauterpacht and Lemkin didn’t know the fate of their extended families from Lviv in Ukraine. Only at the end did they learn that the man they were prosecuting had wiped them - and Leon’s family out.

“It’s difficult to imagine how they felt,” says Sands, who believes his book East West Street (Weidenfeld & Nicholson £20) has resonated “because it tells personal stories and connects them to the big political and international issues right now”.

Sands (pictured centre with Frank’s son Niklas, right) knew little of his family history before uncovering heartbreaking facts about the choices his grandfather made and the losses he suffered.

“My greatest regret is that I never had a conversation with Leon about what he lost and what kept him going.”

Hans Frank was hanged in 1946 for murdering 4 million people.

“It seems incredible that a highly educated, cultured man could do such a thing. His son says it was ‘a lack of civil courage’

“Not doing the right thing when you come to that crossroads is the central question of the book. Frank allowed his identity as a senior Nazi to dominate his individual instinct of what was right. That’s the common theme across the cases I do, people allow their group identity to trump their individual convictions.”

Sands’ event is on Sun 26 at 12.30