Well 7 hours in it's almost gone the first 4/7ths, most of the sky and treetops...almost. I keep designing scenes well beyond the abilities of my confuser to do in any time like the present. Had I the cash I'd be sending all my finals to the farm. I may have to pause the render till tonight as I have some heavy work to do for a friend, and some cash on the computer today. Awaiting that call right now.
update without any of the suggestions as I started the render a while before they were posted...
Mick, the unnatural hole is I think where the SS shader stops on the flattened terrain. In the first iteration it wasn't as visible as the camera was lower. Dunno what to do 'bout it except reset to that camera position.