Finally a decent shot at fixing this. An impressive set of deft moves Andrew – and very glad it worked for you. For me and probably most others, though, the fix fails at the point of needing a boot disc. I just ‘aint got one! So that’s a non-starter right there. Big thanks for posting this up though. If I only had the boot disc I’d give it a go...
MS makes it pretty easy these days to create one on a flash drive. Just grab a 16gb USB drive and download the Windows 10 installation media creator and it will do the rest. After that, it's just a matter of making sure you have Secure Boot turned off in UEFI/BIOS and then boot from the USB drive.
Good luck!