When Adam Ganz was a boy growing up in Oxford, all his friends would speak of what their dads had done in the war. It occurred to Ganz that he had no idea what his own father had done.

It was only many years later that he discovered his father, Peter, had been involved in a remarkable, top-secret operation in which hidden listening devices were used to eavesdrop on the conversations of the captured German generals housed in a mansion at Trent Park in Cockfosters, north London.

All of those charged with translating and transcribing conversations — which contained military information considered vital to the war effort — were Jewish refugees from Germany and Austria, like Peter, who were fluent German speakers.

Adam Ganz, who lectures in screenwriting at Royal Holloway University of London, eventually pieced together the story of what happened when 83 Nazi generals were put under surveillance by a squad of refugee Jews, and the story has become the subject of his play, Listening to the Generals, which was broadcast on Radio 4 this week.

Ganz, whose father died three years ago, says: “From what I read in the transcripts, there was some extraordinary stuff recorded. The generals talked in some detail about war crimes. Some of them said they didn’t understand why the Jews were being killed in the middle of a war when it would be much more sensible to win the war first and deal with the Jews afterwards. Others were repulsed by what was done.”

This information was duly recorded on discs, translated and transcribed by those had fled to Britain to avoid the very horrors the generals were talking about.

One of the surveillance team was Peter Ganz’s friend, Fritz Lustig, who was among those recruited to the operation from the Pioneer Corps. He worked at Latimer House and Wilton Park, both in Buckinghamshire, which also served as prisoner-of-war camps.

Now 90, he vividly recalls his role. “We were told to take note of anything to do with the German army — how it was organised, names of generals, units commanded and conversations about home leave. We also heard accounts from prisoners who had either taken part in or witnessed atrocities against the Jews. This was the first time we had heard of these crimes.”

Lustig says he dealt with this information in a completely professional manner. “It was just one of the things we heard. I don’t think we felt any different about it, compared to the other things we heard. We were just focused on doing our jobs.”

In the quest for information, the generals at Trent Park were treated well — a little too well for some people’s liking. Ganz says: “Churchill complained because the generals were taken on day trips to places like Hampton Court. Occasionally they were taken for lunch at Simpsons in the Strand. The idea was to weaken their morale by demonstrating that German propaganda had exaggerated the damage done to London.”

Despite the fact that some of the generals revealed their complicity in, and certainly their sympathy for, the Holocaust, none of the transcripts were ever used as evidence in post-war prosecutions of Nazis. The secret nature of the operation meant that the information was not obtained in accordance with the Geneva Convention and could not be used in trials. The generals themselves were released after 1945.

However, much of the material was used to aid the war effort. Says Lustig: “I had an interview with a commanding officer on my first morning. He told us: ‘What you are going to do here is far more important than if you were firing a rifle or driving a tank.’ It was a great comfort to hear that because we all really wanted to get into a fighting unit. Still, it was far better than digging trenches in the Pioneer Corps, which was what we were doing before.”

Lustig says that the information they managed to obtain was used to support the famous Enigma code-crackers at Bletchley Park. “Much of the material intercepted by Bletchley couldn’t be acted on because to do so would have alerted the Germans that their codes had been broken. But if there was confirmation from us via a prisoner of war, it enabled the army to make use of it.”

Ganz adds that the transcriptions have also been useful in re-interpreting wartime history. “It is now being used by German historians to expose the good Wehrmacht, bad SS myth. Clearly everybody knew something about what was going on with the Jews and some people knew a lot. That has become historically very important.”

For Lustig, his time at the centre of the operation was intense and has had long-term benefits — he met his future wife, a fellow refugee called Susan Cohn, while working there. They are still happily married now. But despite both being refugees from Germany, they have only ever communicated in English.

“We wouldn’t speak German to each other. In the British Army you never spoke German. So apart from the odd word, it has never occurred to us to speak anything other than English to each other.”