Complex sentence: The Anglo-Saxons obtained control of eastern England in the fifth century but in the mid-sixth century they started expanding into the Cotswold area, then in the seventh century they expanded again into the south-west and the north of England.
Split into simpler sentences: The Anglo-Saxons obtained control of eastern England in the 5th century. In the mid- 6th century they started expanding into the Cotswold area, then in the 7th century they expanded again into the south-west and the north of England.