Well chaps my promised update.
As previous episode - the laptop is behaving like a laptop should - starting and closing down quickly, no crashing, freezing and moving quickly (well enough for me) between screens etc.
So great - except - - occasionally (particularly first thing in morning ?) still got the BIOS screen on start up saying "reset date and time" so off I went into town and followed Rustys advice to replace it and buy a button battery CR2032. Fortunately Dell on this model placed the battery under a three screw panel so after removing main battery and the panel there it was - simple job.
Since fitting new battery approx 10 days ago absolutely first class - no recurring problems - so chaps feel I'm not tempting fate by saying we've cracked it !
Looking back at the saga - - as a little used laptop with an old operating system clearly there was something wrong both with the original OS and the button battery.
My downloading of W10 and not doing a "clean install" exacerbated the problem and mixed the OS's - - it's no wonder it felt "poorly".
Rustys' s advice to run the check disc system gave me confidence the laptop was in decent condition and redoing a clean W10 install solved most of problems - replacing the button battery did the rest.
So once again many thanks - it's great to know you're here to help.
H B E.