After trying many times to get a day-night cycle working, I finally cracked it. It works beautifully apart from a period in the cycle where all the lighting of the geometry goes pitch black. The sky brightness remains with the stars etc, but the level itself goes entirely black. No point lights or spot lights or anything can light the geometry up. To add to that the edges/outline of the geometry go really shifty and jittery against the sky.
This only happens during a period of sun rotation equating to 90 degrees, presumably when it goes below the map. So for 270 degrees the lighting works as I want it to.
I’m pretty sure it’s the light source as when I rotate it manually within the level editor the geometry goes black again within that 90 degree of sun rotation, and when I un-check ‘affects world’ the geometry can be seen again. Switching the option back on makes everything black again.
Pictures below show the difference 10 degrees in directional light rotation makes.