Complex sentence: He worked in an advertising agency until the war, when as an intending conscientious objector he joined the St John's Ambulance; in 1941 he was conscripted into the Non-Combatant Corps.
Split into simpler sentences: He worked in an advertising agency until the war, when as a conscientious objector he joined the St John's Ambulance. In 1941 he was conscripted into the Non-Combatant Corps.