Complex sentence: Nicholls found an easy opening for him at Exeter Racecourse in a 4 runner graduation Chase where Sam Thomas took the ride (Ruby Walsh was injured) where he fell inexplicably made a serious error and unseated Sam Thomas.
Split into simpler sentences: Nicholls found an easy opening for him at Exeter Racecourse in a 4 runner graduation Chase where Sam Thomas took the ride. He inexplicably made a serious error and unseated Sam Thomas.