Thanks guys. Finding a place for the camera is really really tricky because the camera keeps going through the terrain, and even if it looks fine when the preview's finished it still renders with a massive hole in the edge of the image. I did want to have shafts of light like RArchers render, hence why I asked for his guidance specifically but the nature of the displacement at the time made it impossible. I'll try again but it's really difficult to get the canyon that tight while allowing light through and not having massive pieces missing. I'll have a play with FOV too, that might make life a bit easier.
When you say make the cloud layer thin, do you mean almost nill depth or density? Also would you say 2D cirrus or 3d alto, etc. are better?
As for the vertical aspect ratio, I'll try that when I've managed to wrestle the canyon walls into place so I can avoid using narrower FOV's when I can.