Complex sentence: In 1972 he became Ampleforth's youngest ever housemaster, and held for the job for 20 years, becoming in addition the head of history a year or two afterwards, holding that appointment for nearly as long.
Split into simpler sentences: In 1972 he became Ampleforth's youngest ever housemaster, and held for the job for 20 years. He was also named the head of history several years later, holding that appointment for nearly as long.