OK, after applying the latest signature update for Avast and the database update for TF after I arrived home today, I was still having TF flag the Avast scanner. I had not rebooted after the updates at the time as neither program asked for one.
Anyway, I decided to remove the always allow setting in TF that I had set up for the Avast scanner and tried rebooting. Now TF is no longer flagging Avast's scanner. Very strange stuff, I don't get it, but hey the two back working as usual alongside each other now.
Thanks again for your consideration of my problem.