Hi Graham,
Thank you for emailing your file. We are investigating, but for now, it seems that the erroneous shading that appears for some sun angles is caused by intersecting objects in the shading scene. This is a bug that we will try to fix.
To work around this issue, you can try to avoid having any objects in the scene (box, cylinder, roof, tree, active surface) touch each other. For example, if a box represents a building and has a height of 3, its roof should have a z position greater than 3. Also, in your scene, the walkway roof and pool roof touch each other, which seems to cause some shading on the active surface, even when its z position puts it high above the roof where it should not be shaded.
Having objects overlap in the scene makes it possible to more realistically represent what the building looks like, so I realize this not a perfect solution, but I wanted to give you something to work with while we dig into this further.
Best regards,
Paul.