(actually installing Xubuntu but I don't think it makes a difference)
When I attempt to install Ubuntu after setting up the partitions and beginning installation, I end up with a "the 'grub-efi-amd-64-signed' package failed to install into /target/. Without the GRUB bootloader, the installed system will not boot" error and the install cannot continue.
What led to the current situation:
So I previously had Kubuntu installed dualbooting alongside Win10, long story short I formatted the Kubuntu partition, so I've only got Windows installed atm.
Grub then proceeded to break, but with a Win10 iso I managed to 'repair' it (via bootrec /fixmbr, stuff like that). On boot I now get a grub console, if I 'exit' Windows will then boot.
Windows is installed in legacy mode if it matters, not UEFI.