Right-o. I think I'll have to regard this as a long-term project, perhaps for one of my beige relics.
BOAC: Just tried your shields down idea...no joy.
Reverting to Avast v.4.8 would be ideal, but Avast are shortly to cease supporting it (or words to that effect).
My thanks again for all your advice; all noted for the continuing investigation.
Kaspersky Internet Security beckons...