Fixed! though I've no idea how.
Uninstalled AH completely, deleted any reference to AH (folders and stuff); used regclean on the registry, ran the maintenance wizard (scan disk & defrag); downloaded and installed AH1.061 this morning, logged on, recived a very small update and flew.
I'm puzzled, as I run the maintenace wizard at least weekly, so I can't see that that was the cure, but what the hell, I'm flying!