Managed to repair it! Leaving this here in case it helps anyone else with similar problems.
Sata mode was AHCI.
Using F12 for boot menu does not show the usb boot device by default. It has to be added manually in the bios.
Disable secure boot.
When adding a new boot device you give it a name which appears under the (F12) boot menu. Next you select the usb device listed (Easier if there is only one plugged in!).
IMPORTANT PART! Filename -> Don't try entering the filename manually. Press enter and you will be presented with a simple file browser. Browse to efi\boot and select bootx64.efi.
If the filename is not correct selecting the new usb boot option in the (F12) boot menu will just appear to be booting as normal ignoring your selected boot device.
Once I had booted into Win10. Selected repair->advanced and then command prompt.
then reply 'y' to the question. All should complete successfully. Then remove usb drive and reboot. Win10 back up and running.