Complex sentence: This particularly violent headmaster's beating was actually misattributed to Geoffrey Fisher, when the headmaster at that date was in fact John Christie; furthermore the victim in that context was 18 at the time, and had himself abused younger boys.
Split into simpler sentences: This particularly violent headmaster's beating was actually misattributed to Geoffrey Fisher. Furthermore, the subject of the beating was 18 at the time, and had himself abused younger boys.