Well that was a fun morning. Turned the PC off last night as per usual and when I turned it on this morning it wouldn't boot past a "Bad System Config Info" BSOD. Windows auto repair said "failed to repair" and it was stuck in a loop of BSOD > failed repair > BSOD > failed repair. Tried rolling back to a restore point from a few days back, still got stuck in the same loop. Tried using my W10 USB to repair, no luck.
In the end I had to nuke the drive and re-install windows. So now I'm sat with a totally clean install of windows and it's bloody horrible. All my settings and tweaks have gone and now I've got to dig around in the bowels of Windows and go around downloading all the apps that were on the C drive. Just hope my games still work, they are installed on a separate 1TB drive so in theory should be OK and will most likely only require the launcher to be run, but all the settings will be gone if their data was stored in under the Users folder.
Good times.