Complex sentence: The Great West Road to Reading, Gloucester and Bristol was diverted over the new bridge--previously it kept to the north bank and crossed the Thames by ford at Cookham--and medieval Maidenhead grew up around it.
Split into simpler sentences: The Great West Road to Reading, Gloucester and Bristol was diverted over the new bridge. Previously, it had kept to the north bank and crossed the Thames by ford at Cookham, and medieval Maidenhead grew up around it.