The hills are plastered!
I spread a thin layer of PVA glue over the polystyrene, waited for it to get tacky then coated the hills in Wicks Ready Mixed Lightweight Filler. It’s very light, easy to work and it dries to a slightly ‘flexible’ finish.
To complete the job all I had to do was to give it a light sanding to make it smooth.
Thanks to people on the NGRM forum that recommended this filler!