'We recognised the place from the newsreels. There was a large desk covered with dust. An ornamental chandelier hung low from the ceiling. From here Hitler had plotted. Here had been Goering, Himmler, Hess, Goebbels, Ribbentrop - Outside were the remains of a bonfire. The wall was smoke-blackened by the burning. We had reached the heart of Hitler's Germany.' John Mercer was called up in June 1944, and after nine months of square-bashing, vehicle training,...