Update: Exited and restarted program, set to CPU/Low, did a Preview, exported and it did the GPU ~17% thing again and the exported image looked like that blue one above in a slightly longer 2 mins 14 seconds. It doesn’t seem to much like these new drivers.
Update: It did it again.
Update: and again and again.
It seems that the first time after loading the program the CPU/Low export will use ~17% GPU and that will provide a ‘Blue’ image.
If I cancel the export in the middle - as I did after accidentally stopping my stopwatch - then start again it will not use the GPU and the resulting export image will be fine.
But…I then do a Preview and export again and it goes back to using the GPU ~17%, but this time the results were fine. Pixel identical to the previous export. Definitely going back to the previous drivers. 
Definitely going back because now 5.2.0 doesn’t use the GPU at all with the newer drivers. 
Update: had a little panic attack as even with the older graphics driver reinstalled, 5.2.0 refused to use the GPU and Previews were taking 65 seconds whether I turned GPU on or off.
But selecting ‘Use Recommended Settings’ and it did the little ‘Calibrating’ dance and started using the GPU (@100%), so super fast Previews again. Phew!
Nothing exists like that in 5.5.2., so I’m stuck with whatever it thinks is the right (wrong) thing to do with the GPU. Why was this option removed from later versions?
