Ok, I thought I'd post a bit of background purely for interest, as all is fine now.
I originally installed on a development server and posted test msgs. Deleted all except this one. It's the sign-up instructions and usage policy.
Copied the database (by backup then restore) and files across to a production server. All appeared to be working fine.
I renamed a couple of users, but not the admin user with which I had posted the msg, and deleted one of the renamed users (the wrong one) then uploaded the 8.03 files.
So, it may not have been the upgrade. Who knows?