Thanks for the comments.
I won't get a chance until the weekend but now my rendering machine is free I'll make some tweeks:
Improve the grass is a definite

Clouds: I kept the clouds slightly soft but will experiment with quality and or density.
I had already thought that the surfaces could use a little more detail but I don't want to over do things and distract from the shape of the terrain.
Rendering this at all was a nightmare, I tend to develop things on what should be a moderately fast PC; quad core 3Gb ram, while the better spec box does the heavy rendering jobs. For Blender this works fine, TG2 was not a problem but TG3 does not perform well on this machine at all. I couldn't even render the terrain, no clouds, no populations at a detail higher than .3

I did the render no trouble on my laptop, (i3 4Gb), no problem. The big difference is that this machine is running 32 bit Vista while the laptop and i& are 64 bit Win 7 and 8. When it runs 32bit T3 is fine but it very quickly generates errors and drops buckets. I've looked at upgrading Windows, the machine does support 64 bit versions but have been undecided as the cost of upgrading windows and adding memory is half way to a new no frills PC.