Thank you for all your advice Spiritsongs. I think quite frankly I will do as you suggested and monitor Avast myself and wait until the next release of Microsoft is available, to see if that cures it.
I know Avast looks after itself really well. It is something I may find the cause of in Security Center or I may not, but I will certainly keep trying until I solve this problem. If I ever do I will certainly post the cure.
