After delaying for far too long, decided to grasp the nettle and move to 8.1 from Win8. Carefully backed up everything and started the change. Seemed to be going well until, after a few hours, it all stopped, showing codes 0x1900101 and 0x2000C.
Fortunately it then went back to Win8 automatically without me having to restore from backup. Seems to be running OK as before.
Kit is a Dell XPS 8700, just a year old. Compatibility check had only turned up a couple of old progs as potentially needing attention.
Any thoughts would be welcome!