Thanks for the suggestions. Have checked Device Manager and found no exclamation marked items. Nothing found on chkdsk - or rather, I set it to run and when I returned sometime later it had completed & my normal desktop was showing so I'm assuming all was OK. However, I have turned off Windows automatic updates and so far have only had one failure to start-up correctly. May be a coincidence or may be not !! Thanks again.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been