Managed to fit Windows 8.1 on an old Seagate 20Gb drive. I was too lazy to fit an 80Gb one I had spare and thought I would have a go. Was a bit tight installing Win 8 then the Store update but after the update was done the installation is showing 10.8 Gb. Will have to keep an eye on those Windows Update temp files though.
It is only taking up 10.7 Gb on the disc. Since I installed I have found a way to install a clean setup using a MS published generic key, then change to my offical key and activate. MS dont make things easy as the 8.1 disc wont take my Windows 8 Pro key.