Ok, so i did a bit more digging and checked back on some old posts. Found where Series and Season Cover art lives, deep down in users/<myname>/appdata/roaming/j river/media centre <release number> cover art. But i had no Season or Series folders for MC30...... So I copied them over from MC28 (I skipped release MC29), and bingo, all of my old cover art magically reappeared. But there is still no cover art for some of the recently added stuff, like Slow Horses or Bad Sisters. These have TVDB entries, with Series level cover art. I used "Get Movie & TV Info" again, and it picked up the metadata with no problem, bur still no cover art.
So, major problem solved, but I still have 2 issues.
1. Why did MC30 suddenly stop picking up cover art that had been there for a very long time. It was definitely being shown using release 30.0.30, but stopped immediately before 30.0.40 was installed and was not there afterwards
2. Why am I not getting cover art for new TV Shows that I'm adding, even though there is a TVDB entry, with cover art, and I'm finding and picking up the metadata?