Complex sentence: The Bishop of Dorchester was the head of the Anglo-Saxon Roman Catholic diocese of Dorchester, with his seat, or cathedra, at the cathedral in Dorchester-on-Thames in Oxfordshire.
Split into simpler sentences: The Bishop of Dorchester was a bishop in the Roman Catholic Church of England in the Anglo-Saxon period, in charge of the Diocese of Dorchester. His seat, or cathedra, at the cathedral in Dorchester-on-Thames in Oxfordshire.