So I finally got bothered enough to find out what the hell was eating up all the space on my C drive. I had just assumed it was something to do with the enforced Windows 10 upgrade but it turns out it was the AVG Web Tuneup application writing to the AppData directory. (Sorry for blaming you Windows 10)
For anyone else experiencing low disk space that is going unnoticed by disk cleanup utilities it might be worth checking this out. I used the fantastic utility Space Sniffer. This utility doesn’t require an install, it’s super light and super quick and will visually display all the files and folders on the drive you choose to scan. (Tip: run as administrator so it doesn’t throw warnings about directories it doesn’t have access to)
To access the AppData folder on Windows 10 go to start and type %AppData%.
To rectify this specific problem caused by AVG Web Tuneup utility first uninstall it. If this doesn’t get rid of the folder then access the AppData folder and navigate to AppData\Local\Packages\windows_ie_ac_001\AC\AVG Web TuneUp and delete that damn directory.
So long AVG Web Tuneup you jerk!