The problem with only posting within the one other thread that referred to this problem (which, incidentally, offered no solution at the time I posted) is that it may not be read by people who actually have a solution. The title of the other thread was "Avast update brings virus ?", which gave NO clue about the specific problem refrred to. The title of my thread "Win32:Trojan-gen {Other} false positive explorer.exe" made it VERY clear what the issue was - and would be more likely to catch the attention of others who might have had this problem.
Anyway after only 6 hours of wasted time it is finally sorted with the latest update - issued AFTER my original post.
Tony