He arranged to swap for one night at a time with a Jewish inmate he had come to trust. He exchanged his uniform for the filthy, stripy garments the man had to wear. For the Auschwitz inmate it meant valuable food and rest in the British camp, while for Denis it was a chance to gather facts on the inside.
He describes Auschwitz as “hell on earth” and says he would lie awake at night listening to the ramblings and screams of prisoners.
“It was pretty ghastly at night, you got this terrible stench,” he says.
He talked to Jewish prisoners but says they rarely spoke of their previous life, instead they were focused on the hell they were living and the work they were forced to do in factories outside the camp.