David, looking forward to this one. I'd maybe caution against using filler on the foam board as this may cause distortion as it dries. I'd recommend using Gesso, a textured white paint which I have been using to represent rendered walls. Much easier to use with various brushes and sponges for different effects. As you're in the UK I'd go to an outlet of "The Works", the budget book and craft store, where you can get a jar big enough to do your entire building I would guess, for around £10, much cheaper than in regular art shops.