Well I've been working on a scene to put the car in and I think I'm most of the way there. There are a couple of problems however.
The good ol' GI is infecting the clouds with its bucket miss matches, I thought this had been fixed. Could this be something to do with the cloud layers overlapping?
The road repeats itself, I need its mask to make the road surface, if I don't tile it the road just appears as a nice layer about 2km away. Guess the answer is to simply go for a very much larger mask, one that stretches as far as can bee seen, about 13km in this case (13000 px) so not impossible.
Lastly I'm not too happy about the shading on the desert grass plant (homemade). It dose not de-saturate properly with distance. Could I make a distance shader run the saturation in some way? Don't know about that one.
The red beach ball is just a stand in, to check if I'm getting the right lighting in the foreground.
C&C and any ideas for solutions very much appreciated, thanks
Richard