Kingpin
You can hear both the version used in the cartoon and Jimmy Black's rendition at the audio project… LINK They're both notably different in how they sound even if the Black rendition uses the notation of the cartoon theme… which I felt had a bit more punch to it.
If I recall correctly, the end credits of RGB said the song was sung by a ‘John Smith’, which really helps us in identifying anything…
'John Smith and the Common Men', probably…