"Run to the hills....."
The hills are all but finished now - they just need a sealing coat. Given my experience with spraymount adhesive (leaves the surface very sticky for ages), and using a dropper to apply PVA (seems to mess things up) I've decided to bite the bullet and get some scenic cement and a sprayer tomorrow.
Pics showing the last three stages:
After dry-brushing with Cool Grey matt emulsion
After adding patches of sand/ore/ballast mix
After adding flock