Thanks XRaySpeX.
I didn't post yesterday because I loaded Task Manager 40 minutes after a cold boot, having done nothing but log in to Windows. It was showing 191+ hours CPU time., and all the other rows on the top screen were in hours as well. So I concluded for some reason it wasn't clearing down.
Which is what you have just explained!!
So instead of trying to find out from scratch what is going on I have a starting point from you.
IIRC I did do a restart for some other reason, or it may have been a hard switch-off, and that reset it. This system does seem to hang at times, but in an odd way. Not always completely - for instance IE11 may stop responding but will close, and on re-opening works - but slowly. Similar other effects. Thunderbird similarly.
Only "disc" is SSD and fast boot appears to be on. Though booting from SSD on a machine I sent back was even quicker - maybe that wasn't saving the state.
Regarding saving the state, that was one of the clues I had found. Closing IE with tabs open before closing down always results in not just the tabs re-opening as per my options setting, but with the previous contents not the current.
Anyway, another busy day ahead, so I shall not be investigating for now. Thanks again for your input.