This was easily fixed - I just created a new Dsub server with exactly the same address as the original and it immediately picked up the new genres.
However, I don't think this should not have been necessary - what is the "correct" way to re-sync the genre decriptions between PC-based subsonic server and Android-based Dsub client? Or did I just not wait long enough

Edit: I should have added that I browse by folder and not by tag
regards
David