I think the overheating was due to Windows Update trying to find them but being baulked and I also found TrustedInstaller at the top of the list for max usage.
My laptop got so hot that it was uncomfortable to rest my fingers on the touchpad.
I think after my experience and resolving, I now have no fear of WU hanging.
In reply to Willy2, on the one I'd factory reset, yes it did.
On the one I'd gotten back, when I'd fixed the update problem and they were presented, Win 10 was the main one in the box and I had to click on the blue link underneath to view the other updates.
On that one when the icon was first issued last year, I went into the registry and did a search for and deleted GWX as well as deleting the trigger for it in Task Scheduler, so I only occasionally get the Win 10 one mixed in with the rest of my updates.
When I tried to do the same on the factory reset one, I found that I was unable to remove the registry GWX because Trusted Installer owned it and any option to change ownership was greyed out.
There was also nothing in Task Scheduler that I could delete either - it seems that MS must be getting desperate.