Complex sentence: Derek was a washboard player in a skiffle group in 1956 and later a drummer in a rock'n' roll band, after which he spent 10--18 years working in the accounts department of the confectionery firm J. S. Fry & Sons (Cadburys).
Split into simpler sentences: Derek was a washboard player in a skiffle group in 1956 and later a drummer in a rock'n' roll band. Afterwhich he spent 10--18 years working in the accounts department of the confectionery firm J. S. Fry & Sons (Cadburys).