The series suffered from the BBC's lack of a proper archiving policy until 1978. It was BBC policy before 1978 to wipe master tapes and reuse them for other programmes, hence saving money and storage space. From a full tally of 799 editions, 466 (broadcast between 1962 and 1974) are thought to no longer exist; additionally, some colour videotaped episodes only survive as monochrome film copies. The final four series, broadcast between September 1974 and September 1978, exist in their entirety.