As a quick update - rather than having to sit through listening to me in a video, which should probably be classed as cruel and unusual punishment - I've added a few summary pictures, including close ups of the textures, to the blog post instead. I've also posted (hopefully) the main picture (below).
My recipe was: 20g plaster of paris, 10g water and between 15 - 35g of fine(ish) sand (top row).
Adding 2.5ml of pva made it feel slightly less brittle (bottom row).
The greener bit in the top right is the vallejo earth texture, for comparison, which looked pretty good too when it had finally dried.
I'd still like to hear of any other "recipes" if anyone has them.