Some slight confusion here. Peter Butterworth was indeed a POW in Stalag Luft III, but he was mainly involved in a different escape attempt - the one chronicled in the movie "The Wooden Horse". This was where a group of prisoners practised gymnastics in the middle of the compound, with the entrance to the tunnel being concealed beneath the vaulting horse - while the German guards watched the atheletes, they were unaware of the man inside the horse digging the tunnel. When the film was made, Butterworth went up for the part of one of the gymnasts - basically to play himself - but was rejected on the grounds that he didn't look athletic enough! He'd probably put on some weight in the intervening years.