Fixed! I think 
I have said that before. However, now 2 hours without a black screen period at all. I tracked it down, I believe, to a glitch in the MSI Afterburner/RivaTuner which, rather than try to fix, I uninstalled - with some difficulty - but they are currently gone.
I did do a clean install of the AMD graphics drivers, although I didn’t use DDU. Reading the DDU instructions I thought wow, there are a lot of caveats here, also it did say that it should be used only if the conventional uninstall methodology didn’t work. So I went down the Radeon Adrenalin route first and that worked fine. Latest driver installed - no change, still getting black screens for 3-5 second intervals.
Removing the MSI Afterburner and RivaTuner has produced a performance hit with FPS dropping down to 35-40, GPU utilisation from a constant 99% to fluctuating between 50-95% and all temps down by 10 - 15 degrees. CPU, RAM and VRAM utilisation hasn’t changed much. I haven’t tried tweaking anything yet but I think that what I will do is to fly for a while and confirm stability and once I am happy that it is working ok, re-install the MSI Afterburner.
My thanks go to all of those who answered my cries for help, especially to MichaMMA and BubbaBlitz who helped to focus my less than knowledgeable efforts.