Complex sentence: Montrose used biplanes for training fighter pilots right up to the Battle of Britain but they were about to be replaced by Miles Masters which had a performance like a contemporary fighter, supplemented with a few early Hurricanes as they were withdrawn from front-line service.
Split into simpler sentences: Montrose used biplanes for training fighter pilots right up to the Battle of Britain but they were replaced by Miles Masters which had a performance similar to contemporary fighter. They were supplemented with a few early Hurricanes as they were withdrawn from front-line service.