Complex sentence: In 1950 he became a member of the Johnny Dankworth Seven, with which he played until 1953; it was during this time that he first achieved fame, but when Dankworth decided to put together a big band, Rendell parted company with him.
Split into simpler sentences: In 1950 he became a member of the Johnny Dankworth Seven, remaining until 1953. When Dankworth decided to put together a big band, Rendell parted company with him.