Thanks for your help Asyn.
In short it seems to be working now.
Details:
Avast cleaner would not run in safe mode, or safe mode-command prompt, kept complaining about a stub not able to run. So I tried in 'normal' mode, despite warning against it, it seemed to complete successfully.
After reinstall/relicence/reboot, Avast came up in passive mode - which I didn't know about. It did that because of Malwarebytes. I don't really want Avast as scan only, so I took passive mode off, restarted, and all seems well for now anyway. I know plenty of people advise against more then one AV pgm, but I find the risk of a conflict worth it for the extra protection (probably) afforded. I've never had a problem with them conflicting - until now, maybe.
I'm not sure if there was a conflict, but they both seem to be running happily now.
I would still like to know if anyone else has had similar issues...
Thanks again.