Thanks again, Dave.
I have saved all relevant profiles.
...completely uninstall and reinstall Opera 36, using the standalone installer from here.
Got the file OK, but:
After uninstalling, clean the system of all Opera 36 folders (be careful not to delete your Opera 12.18 folders!) before reinstalling.
That is a problem - as they all seem mixed together. 12.8.exe is Feb 2016, 36.exe is 8/2016 but there are plenty of much older folders in there back to 2010 (inc OperaBackup)
Should I try an uninstall/reinstall without cleanup, or could that make things worse?
Bit of a pain at the moment as I can't get to so many of my bookmarks/passwords etc until I restore my old profile. Like having one hand tied behind my back.....