The de-flocked board dried fine, and actually doesn't appear to need re-undercoating black. A quick run out to B&Q sourced the Holy Grail of terrain undercoats - Sandtex Bitter Chocolate, as promoted by the Mighty Dux himself, Rich Clarke. (Why it's so good I'll leave for the next Colour Concepts article!).
I have some black stone-textured spray paint (also from B&Q) which I used to overspray the filler that was hiding the skulls, as well as one of the more egregiously fantasy sections of cracked stone roadway. Only drawback with this was discovering it takes 24 hours to dry hard, so I wasn't going to get to wield a brush and the Bitter Chocolate, which was a shame as the sun came out.
Never mind. Next update not for a bit as there's this tedious thing called work...
Almost forgot - placed a big order with War World Scenics for lots of suitable grass.