i'd say if you don't mind waiting for a while. use a standard skylight and make it very dark and a bit blue. just create a skylight, then you can adjust the light tracer settings to make it a bit faster (lower rays/sample) then add a target directional light, make it white and adjust its multiplier so its not too bright. use shadow maps. and under shadow map params make size roughly 1200 and sample range 20.
that will give you quite detailed shadow maps but blur them out quite a bit too. That way you have a directional light for the key "moon" light, directional lights are good for the moon or the sun because the rays are parallel simulating a very distant source. then the skylight with take care of any bounced light from the environment. Thats just one approach maybe it will work well. its a pretty simple way to get GI happening without moving to mental ray or vray.

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Been playing around with it, and got a nice look going, thanks. Taking a bit of playing around to get the look im going for, but its much nicer than what I had before.

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My latest 3D project:
[Link to ja3dface.blogspot.com]