Complex sentence: By the middle of the 19th century the population had drifted away to Sutton (modern day Selsey), largely because of coastal erosion, it was therefore decided to move the church to the new centre of population.
Split into simpler sentences: By the middle of the 19th century the population had drifted away to Sutton (modern day Selsey), largely because of coastal erosion. It was therefore decided to move the church to the new centre of population.