@jumper
Just an addition to what I said yesterday, I did notice later in the day, quite a few days after I copied the 4.5.2016.20 kexbases.dll and core.ini files across, that Opera 12.02, although apparently working fine, was not showing all of its menus.
This turned out to be because the KernelEx mode on opera.exe had for some reason reverted to "Base Enhancements (api fixes + extensions)".
When I returned it to "Windows 2000 SP4" which is what it had been, everything came good again.
I just thought I'd mention that in case anyone else hits the same problem.
I'm not sure why the mode should have changed.