Yesterday SWMBO got up and, being on autopilot at that time of day, turned on the machine and carried on as if nothing had happened. The machine worked completely normally.
Later I interupted her progress and ran my usual scans (Spybot, Ccleaner) and nothing showed up. When I launched the spreadsheet that seemed to trigger the problems the previous day it was just an empty file and when the data was re-entered nothing untoward occurred.
I was going to do the sfc scan but noticed that the machine has an SSD and I ddn't know if the test would be good/meaningful/harmful?
We have not been able to cause the issue to recur but will keep an eye on things. At least we have been prompted to do the first proper backup since the machine was bought.