Well yes, the jpeg linked in the post would indeed be a challenge as it isn't a linear randomization! I only put that in for the colour example really, but it would be a great challenge if someone could take it up.
I'm thinking of making 5 or 6 variations of materials in a multi-sub material, and then try and get a random mat id going in railclone.
The other thing I wanted to ask was how do you make this sort of pattern work on a non-planar surface, i.e. a terrain. Obviously I'm trying to make a private drive, and it is not coplanar at all.
Thanks to Rokas for your reply, but unfortunately I am using vRay (only unfortunate because your suggestion doesn't apply).
I really hope someone can help with this soon.