Well, it seems that CurrysPCWorldDigitalDixonsComet weren't prepared to write it off, so back I went yesterday to collect it - with its new hard drive.
"It still has Win8.1, please re-install Win10!"
"Certainly sir - should only take 30-40 min"
So I went for a wander around the store. Lots of very nice toys!
40 min later it was still updating...
1:40 later it was still updating...
2:40 later it had done its initial Win10 update, but was downloading yet more Win10 updates...and had been at 33% for about 30 min.
"Do you want to take it and let it keep updating, sir?"
"No - I'd like you to check the update and make sure everything is OK, then ring me to let me know when it's ready"
"Certainly, sir"
So we'll see how it goes this week...