I investigated the sort triangles option, which was only available to me on static mesh components, I had only placed a SM from the content browser in the world. Wonder why it isn’t available there.
The artifacts have disappeared, but I still don’t really understand the “Triangle sorting” view of this. To me is looks like there are clear bands running down the mesh in the X direction, almost like a series of parallel planes have intersected and shaded the regions under the mesh.
But its gone now and you have my gratitude!