Damn - my fault. I had put 7.2.2 in a new directory and copied in the styles, includes etc. rather than copy the new version over the top of 7.1. The upgrade script didn't complain though, which seems strange...
I just this minute got it working by taking a copy of my 7.1 tree, copying the 7.2.2 files into it (after having removed the necessary directories from that tree), re-running the upgrade script (which straight away said it had finished) and it kicked in again fine.
Phew!! I shall get to sleep tonight after all

Thanks for the prompt reply.
- Paul