Philippe Sands in his Hampstead back garden: he is frequently reminded of the Pinochet case as he can see the very room the warrant for Pinochet’s arrest was signed by magistrate Nicholas Evans whose former home overlooks his house

DESPITE evidence of mass murder, General Augusto Pinochet flew to the UK to have a minor procedure at a posh private medical centre, The London Clinic.

It was 1998 and the former dictator felt secure enough that when Met Police officers arrived at his bedside, he was unpleasantly surprised: they had a warrant for his arrest.

Spanish magistrate Baltasar Garzón had seized the opportunity to slap handcuffs on the man who led an armed coup against Chile’s democratically elected government in 1973 – and was responsible for the murder of thousands of people.

Law professor and barrister Philippe Sands represented campaign group Human Rights Watch in the Pinochet case. And he now tells a story that intertwines the dictator with a genocidal Nazi who found a haven from international law under Pinochet’s regime.

The book reveals that Philippe was asked to represent Pinochet and as a believer in the cab rank principle – that barristers take the next case regardless of personal beliefs – he was duty bound to fight Pinochet’s corner.

His wife, who is Spanish, told him she would seek a divorce if he did: instead, he took instructions from Human Rights Watch.

The book, 38 Londres Street, is the third of a trilogy. Philippe chose the title after the building in Santiago where Pinochet’s death squads dragged opponents and tortured them: it had previously been the centre of a Socialist party before the coup.

Philippe’s previous works, East West Street and The Rat Line, investigate the links between his family,  the Second World War, and how Nazis escaped justice.

“It started in 2014 when I was researching The Rat Line,” he recalls.

He came across an intriguing letter in archives: it was from a man called Walter Rauff to another Nazi, advising him to head to South America.

Augusto Pinochet [Ministerio de Relaciones Exteriores de Chile]

“I saw that Rauff had fled to Chile,” he says. “I saw there could be a link between him and Pinochet.”

It prompted a decade-long detective story – and reveals, for the first time, a link between one of Nazi Germany’s most wanted and Pinochet.

And as with East West Street and The Rat Line, Philippe could not ignore a personal link.

Carmelo Soria, a cousin of his wife, was one of Pinochet’s victims.

He was asked by a Chilean judge why he had become so interested in Rauff: “It was a decent question,” he writes. “It began with the law, how the crimes created by the Nuremberg court were taken forward, the interplay between immunity and impunity. There was also a personal connection through Carmelo, and I discovered that one of the many murdered in Rauff’s vans was Herta Gruber. She was 12 years old, my mother’s older cousin.”

It reads as a detective thriller rolled up with international law, sover­eignty and immunity, and the moral question about an responsibility in the political sphere.

Philippe interviews scores of people who offer clues and personal remin­iscences – and the legal figures and politicians who were involved in the arrest of Pinochet.

Inspiration came from a neighbour. The late writer John le Carré lived opposite Philippe in Hampstead.

“He was fascinated by Rauff and had a role in shaping the book,” says Philippe. “Le Carré explained how to take a complicated subject and make it readable.

“He explained how you place little clues. The reader understands and wants to work out the puzzle.

“John had a deep respect for the reader not immersed in the world of espionage – or in my case, the law – but who understood the world.”

As Philippe’s research shows, Rauff, who was responsible for Nazi murder vans that used exhaust gas to kill victims locked inside, became the manager of a Chilean crab factory. His past was well known. There had been attempts to hold him accountable, and cases were prepared for his extradition. He lived in fear of kidnapping or assassination – and Philippe reveals he narrowly escaped Israeli agents.

Walter Rauff

While Rauff claimed he was innocent – he would say he was “following orders”, Philippe has uncovered evidence that shows this rabid, anti-semitic murderer continued his work. Without giving too much away – the book deserves no plot spoilers – Philippe sets out to establish a link between Rauff and Pinochet. The results are astonishing, and he expects the findings will cause consternation in Chile.

“It provides evidence for the first time of the connection between crimes in Latin America in the 1970s and 1980s and one of the perpetrators of Nazi war crimes,” he adds. “It shows that these individ­uals did not melt away. They continued elsewhere.”

Philippe spoke with many sources – including Pinochet supporters.

It was not easy. “If you treat them respectfully, prove you will not alter a single detail, trust is built,” he says. “In a courtroom, political discourse is not tolerated. You share a space with people who you may violently disagree with but you treat respectfully. I learned from le Carré not to lead the reader. The reader, like a judge, wants to form their own view.

“The job is to lay out the material faithfully. That can be difficult, but you let the reader decide. A barrister respects the judge to come to their view – as a writer, you have to respect the reader.

“And in these times of fake news, I think the role of the writer is to fairly lay out the facts, and not push an agenda.”

The book reaches back to the Nuremberg trials and its impact today: with Vladimir Putin and Benjamin Netanyahu accused of war crimes in Ukraine and Gaza, international law is in the spotlight.

“We are living in a moment where there is a challenge to ideas put in place since 1945,” he adds.

“I am not starry-eyed – they have to evolve – but the fundamentals remain. Abuse of the law and high office does not exclude you from being held to account. It was revolutionary in 1945 and deserves respect and protection today.”

And as Philippe points out, the arrest of Pinochet had a global impact.

“It caused others to reflect on their own immunities,” he says.

Croatian President Franjo Tudjman decided not to head to Germany for urgent medical treatment – he faced indictment for crimes in the former Yugoslavia. George W Bush called off a visit to Switzerland, said to be because he feared being interviewed about waterboarding, while Putin missed a summit in South Africa following crimes in Ukraine.

For many families who mourn loved ones in Chile, there has been no justice and no closure.

When Pinochet died in 2006, he was under house arrest and had been indicted. As with Rauff, death meant he cheated justice.

• 38 Londres Street: On Impunity, Pinochet in England and a Nazi in Patagonia. By Philippe Sands. Weidenfeld & Nicolson, £25