The first two series were first released on record (as broadcast radio in 1979), audio cassette (revised from radio) and CD in 1988, marking the tenth anniversary of the first broadcast of the first episode. These were the first programmes of any kind released on CD by the BBC Radio Collection. The two radio series were known simply as "the first series" and "the second series" until 1992 when the BBC made its first re-release in separate boxes as "The Primary Phase" and "The Secondary Phase". The episodes were released with those titles in 1993, and again in 1998, for the series' twentieth anniversary. In 2001, they became the first programmes of any kind re-released by the BBC Radio Collection in an MP3-CD format.