Strange blocks in Atmosphere render

Development Rendering
,
1

I’m getting strange random “blocks” appearing on my renders in the atmosphere.
I’ve attached two images showing the issue.
I’m using Ultra Dynamic Sky as my only light source.

Hills_08.0000
Hills_08.00001920×1080 1.61 MB

Hills_08.0012
Hills_08.00121920×1080 1.61 MB

The “blocks” radiate from a central point, which may be related to some volumetric setting.
I also notice a fully “black” version of the “blocks” flashing up during the render preview (which I’ve not been able to screen cap as it happens too fast!)

Any ideas for a solution or a cause.

Thanks
R

EDIT: Just tried this again without using Ultra Dynamic Sky and just a vanilla Exponential Height Fog and I am still getting similar results and

Hills_12.0000
Hills_12.00001920×1080 1.34 MB