Complex sentence: Barnstaple was formerly referred to as "Barum", from a contraction of the Latin form of the name (ad Barnastapolitum) in Latin documents such as the episcopal registers of the Diocese of Exeter.
Split into simpler sentences: Barnstaple was formerly referred to as "Barum". The Latin form of the name (ad Barnastapolitum (?)) was reduced to ad Barum in the Latin documents of the time such as the episcopal registers of the Diocese of Exeter.