Complex sentence: In Stalag IV D he certified a number of prisoners as insane, who were consequently repatriated, and then convinced the Germans of his "own" insanity and returned to Britain the same way.
Split into simpler sentences: In Stalag IV D he certified a number of prisoners as insane, who were consequently repatriated. He then convinced the Germans of his "own" insanity and returned to Britain the same way.