Reporting back on this issue....
Just a cursory look through the Program Files (x86) folder turned up some past Opera files, which had somehow missed the previous uninstall process. I manually deleted the parent folder and contents myself.
I then installed and paid for Revo and used the 'Forced Uninstall' tab to look for 'opera' leftovers in the registry keys. This found numerous entries under Client, News, Mail and other Microsoft sub-headings. I selected all components that specifically were sub the "Opera" name in the tree and after creating a restore point, then deleted them.
This step appears to have worked without damaging anything else. I've opened and closed routine programs, shut down and restarted etc. without noticing anything amiss.
However CCleaner still reports Opera in the Apps list. Ha! I'm now wondering if I need to wait until the next upgrade to overwrite the old Opera settings?