So I finally got Windows 10 Installed!
After maybe 15 failed attempts with various setups, every single time getting that C1900101-20004 error, I managed to get my hands on an old VGA cable (native motherboard graphics has no HDMI output) so I could completely uninstall and remove my Asus (Nvidea) Graphics card. I nearly had a heart attack when copying files went to 85%!
Now reinstalled Graphics with Windows 10 drivers and got the network card back in too using the Troubleshoot tool to install in Windows 7 mode. Working perfectly.
Not sure if it's worth all the hassle just yet, though it is nice to have a (native) start button back, and Cortana is worth laugh too for all of half an hour.