AAAAAAAAARGH!!!!
I got the iLiad working again, backed up my annotated PDFs, re-set up the WiFi connection so I could upgrade to 2.12 - and now it's back in the reboot loop
OK, I'm going to try the "format" option - if that doesn't fix it, I'm down to sending it in to be fixed, I fear...
EDIT: The reformat seems to have worked (fingers crossed) - I connected via my wired network and left it to update, and I have 2.12 working again. Now to reinstall all my documents and set up WiFi again