Thanks for the suggestions - this is starting to feel like a "bad seed" kind of problem.
I'm doing many, many tests now, changing one variable at a time. Have taken out all plants, all fake stones, and taken out all surface group shaders except the base color. This leaves only the terrain block . It uses only the default nodes, but with another seed and a smaller displacement value and little different simple shape .
Even with only the basic terrain enabled, the banding still appears at GI phase. I've run about 30 different variables so far.... like sun position, ambient occlusion instead of global illumination, GI levels, gamma, fall off, occ weight, ray detail, camera lens fov, prepass padding, etc.,etc.
Will report my results when I come to some useful conclusions.