Thanks for the suggestions.
I did set Opera up using the Single User option (ie. myself/ administrator) rather than using the default settings and I was thinking something about this might be the cause of the problem.
Before I came back here and read your posts I acted on this suspicion. I thought the most sensible thing to do then was unistall Opera as I’ve only just started using it. This was less trouble than the more complex changing Opera’s User Prefs. paths from Single User to All Users. So I used Revo Unistaller to make sure everything had gone and reinstalled Opera but this time for the default All Users.
Yay! Just checked and it is now being recognised by Avast. I know the Stand Alone option has issues but there was no mention anywhere I looked about the Single User install having them too. But why does Avast do this when Opera was installed both times in the same C:\Programs\Opera folder? Is Avast only looking in the All Users programs folders?