Complex sentence: Johnny left Pipkins around 1978, to be replaced by Tom (Jonathan Kydd), who moved on in 1980, his place taken by Peter Potter (Paddy O'Hagan).
Split into simpler sentences: Johnny left Pipkins around 1978, to be replaced by Tom (Jonathan Kydd). Tom moved on in 1980, his place taken by Peter Potter (Paddy O'Hagan).