Thank you very much for the quick answer!
It does indeed seem to be caused by distance field lighting, thought I couldn’t find the exact settings to get it working nicely so I’ve disabled “Affect Distance Field Lighting” and that has removed the distortion.
I suppose it’s due to my scaling up of this mesh and landscape and on larger meshes I’d want proportionate resolution, though I didn’t find the precise settings to do so without re-building my scene’s lighting. Will give this a bit more research and update when I find the proper settings to adjust.
Thank you.