Problem with Landscape tiles


I have a landscape that uses three materials plus foliage. After building Lighting I notice a few of the landscape tiles appear to be incorrectly shaded and the tile straight edges show up clearly as illustrated in the captures below.

I’ve seen elsewhere a recommendation that setting Sky Light from Stationary to Moveable cures this and it does remove the visible tiling but the overall lighting quality appears much poorer.

Can anyone tell me what I am doing wrong and how to fix this?

Many thanks!