So while I'm now pretty happy with the shape and pattern of the cobbles I've been trying to create I wasn't entirely happy with my first attempt at painting them. While the effect looked quite good it was very dark, and I think would have looked very wrong if used on a larger area, so I've had two more goes at getting the colour right.
I think the left hand side (black base coat) now looks much better. The use of the lighter flint grey and cork brown have improved the contrast between the cobbles and the gaps between them and I'm really happy with how it looks. The right hand side, however, definitely isn't right; the gaps between the cobbles just aren't dark enough and there isn't enough contrast.
So I think I can call this experiment a success. It may have taken me two attempts to shape the clay and two to paint the cobbles, but I'd much prefer to do that now with small amounts of track, clay and paint then on the full layout where having to start over would have been very wasteful in terms of time and money.