Back in the early days when Southern Sound and Radio Mercury shared evening programmes (eg. The Timbo Show) the commercial breaks on each station would often be different lengths, so they would play library(?) music to pad out the shorter break.
Examples of the 3 tunes used can be heard here:-
http://ssaunders.dyndns.org/ilraudio/ss/ss1rm1addfill.mp3
Bit of a long shot, but I don’t suppose anyone can identify the tracks?