Update on the Windows PC situation:
I tried @matigo’s solution with the command prompt and that didn’t work. Tried some of the other solutions offered by several YouTube videos, system restore, system image recovery etc to no avail. So I’m thinking ‘it can’t be as simple as a clean install, right?’. I’d already tried to boot Windows 10 before and it didn’t work.
So at close to 11pm last night, I’m sitting on my bed installing Windows 10, waiting with anticipation as more errors come my way. Nope, no errors. Everything worked. I have a PC back now, for the moment. There’s a sale on at the moment and I’m looking at getting a gaming laptop. I haven’t fully decided yet but at least being able to fix this one has bought me some more time to really think about the upgrade.