Just to say that I seem to have solved this issue (for now at least) with the help of these people this morning.
http://forum.avast.com/index.php?topic=76528.0
I'd recommend them to anyone with Avast issues.
Oh, just to add uninstalling/reloading the programme seemed to do the trick so perhaps there was a glitch in the system beforehand. I'll be keeping fingers crossed for the next few weeks though as I have a suspicion that the problem was due to an error relating to the 30 day registration period. I did successfully register the software the first time I installed it and but it's as though the system didn't fully recognise that somewhere along the line.
Anyway we'll see if it happens again....