Complex sentence: At the beginning of the 20th century Bishop's Cleeve had barely 400 people, but by the 2001 census this had increased to 9,944; mainly due not to the close proximity to Cheltenham, but to the arrival of major employers in Bishop's Cleeve itself.
Split into simpler sentences: At the beginning of the 20th century Bishop's Cleeve had a population of barely 400, but at the 2001 census the figure was 9,944. Yet this increase has been mainly due not to close proximity to Cheltenham, but to the arrival of major employers in Bishop's Cleeve itself.