Hi Bob
As I mentioned, its the actual 'log' file in the spamconf folder. The log file is over 2GB in size and getting bigger. I have since seen this problem mentioned elsewhere on the forum. Apparently this is an known issue and has supposedly been addressed by avast in recent updates.
I dont know enough about it to start messing around in there but I need to get this sorted otherwise its just going to carry on chewing HD space.