The laptop is a Samsung NP350V5C.
I have the hard disk out of the machine to allow interrogation by my Linux machine using a caddy.
The working one prompted me to install updates last evening and I let it, thinking it would be ready to play again by the time I had finished my dinner. NO. Two hours later it was un-installing the updates as it had failed to re-boot the first time. It did this twice so I left it (as advised - do not switch off) and went to bed. Seems to have sorted itself out as it is working this morning.