Complex sentence: He was allowed to return to the United Kingdom in October 1941 he then joined the Royal Pioneer Corps in which he worked for eighteen months before he was allowed to join the Special Operations Executive in 1943 due in part to his being a native German speaker.
Split into simpler sentences: He was allowed to return to the United Kingdom in October 1941. He then joined the Royal Pioneer Corps in which he worked for eighteen months before he was allowed to join the Special Operations Executive in 1943 due in part to his being a native German speaker.