I checked online on Jen's computer to see if there were any clues on Google and there were some looping issues back in August... but they were fixed.
The only option seemed to be to switch off and switch on again - reboot. I did so and Windows then went into reconfigure mode for about half an hour and then it was 'Welcome to Windows 10' and, touch wood, all is plain sailing, all files present and correct (backed up anyway). Phew!
I'm now busy doing a final final edit of Cataclysm, the third 'Avenging Cat' crime thriller due out from Crooked Cat Publishing later this month.