Sixty years ago, on April 15, 1945, Lieutenant John Randall, then a 24-year-old SAS officer, was on a reconnaissance mission in northern Germany. He and his driver were heading down the road to Lüneberg when he noticed a large, imposing iron gate in front of a track leading off into the woods to their left. Curious, Randall decided to investigate, and so discovered one of the most horrifying aspects of Hitler's Germany. "We were totally unprepared for what we had stumbled across,'' says Randall, now 85, sitting opposite me in the Special Forces Club in London. ''I just drove through...