Interestingly enough I decided to give Avast 9 another try at the office (separate license) and noticed that the file size of the installer had changed.
When the original installer was downloaded on the 16th of October is was 121,506,128 bytes, but when downloaded today it is 121,680,752 bytes - 1.74624Mb bigger!
I'm therefore wondering if the installer has been updated to fix Windows 8.1 install issues?
Fingers are firmly crossed and I'll try it at home tonight!