I've now found my way through to "Graphics properties", a window I've not discovered before, and have an explanation of at least part of the symptoms.
The facility of having both screens on is one the options, and inexplicably it became the chosen one. This explains why the F8 key was having no effect !
I've now switched to just external monitor, and that works, but still only with the 3 hour warm up.
So I shall proceed with an examination of the display module, but more importantly, see if the laptop will boot up without the display connected.
I planning to load Linux on it, and a remotely mounted screen would work well for what I want to do with it, so that's the idea.
However getting it to boot up properly would be preferable to leaving it permanently on.
Switching to standby doesn't bring a quick reboot, by the way.
John