Complex sentence: As Watkin's practice grew he took on a partner Mike Booth, then John Clifford who initially assisted at Watkin House, then in 1959 became Watkin's partner and finally, in 1965, his successor.
Split into simpler sentences: As Watkin's practice grew he took on a partner Mike Booth, then John Clifford who initially assisted at Watkin House. Then, in 1959, Clifford became Watkin's partner and finally, in 1965, his successor.