Unfortunately this has started to happen again, and this time the troubleshooting is not coming up with anything. So annoying as it means if I leave apps open ready to start back on them again, they have all gone next time I start the machine up. Back to the drawing board!
The instructions for safe mode cannot be followed - "General" does not exist on PC Settings menu.
I have looked at the event log for this morning and it had an entry
Windows failed to resume from hibernate with error status 0xC0000411.
Shortly followed by
The previous system shutdown at 22:59:28 on 30/10/2013 was unexpected.
The settings are set to Sleep on lid close and power button, both on plugged and battery. Then set to Hibernate after 60 mins battery and 180 mins plugged in. Last night the lid was closed and left on power.