To finish this thread off the problem got worse and the computer would not boot up at all other than in safe mode. Left it with a computer repair firm and in the end the only solution was to restore the OS. Thankfully safe mode allowed the savings of data.
I think the problem started with the shut down mentioned in my 25 Sep post. Prior to this when you switched the computer on at the mains there would be a quick burst of the cpu fan. During the problem period this didn't happen but following the restore it is doing again. May mean something.
Needless to say I've switched off the sleep function.