Ron I have found a block of polystyrene and tile grout (floor or wall) a good combination for making various walling effects . Start by cutting a polystyrene block to about the size and shape you want to end up with , pin to the base (I use cocktail sticks and PVA)you need then coat the polystyrene with PVA , while it's still tacky cover with tile grout , for brickwork about 2mm , for stone about 5mm . After half an hour or so you can start carving into the surface creating the effect you want . If you are not happy you can smooth down a bit with a damp brush or wet it up and add more grout . The great thing is you can play with combination for a couple of hours.
And when it's all dried out , which dose not take long you can paint with any paints you prefer .
chippy