Problem solved. As I was looking around the PC trying to work out what the issue was, I found several Windows functions not working - for example right click on the start menu - Control panel wouldn't open (although settings and device manager were OK). So I started looking for a solution to that and ended up restoring Windows to it's out of the box state, saving personal settings only.
I then went through all the sleep settings again etc , reloaded WSJT-X, JTAlert and DXKeeper, and Dimension for the timing, and everything is working now. Just don't know what caused Win10 to go faulty.
Thanks to everybody who has contributed advice.