It was specially written for the series by Granada's resident musical director Derek Hilton under the name John Snow - some of Hilton's other scores for Granada shows were also released by de Wolfe, including Spindoe and Flower Of Gloster.
Presumably this was done under one of those arrangements where the library made the recordings for the show, and were given the rights once the show was over, but who knows? Peter Taylor, who was one part of the Peter Reno writing partnership with Cliff Twemlow, worked for Granada so I wonder whether if he oiled the wheels there?