I've now painted the house with a 50/50 mix of PVA and water, which gives it a good base for painting. Without it, any wallpaper or paint may not take.
Just above the edge of the roof, I've glued three barbeque kebab skewers, with the ends cut off and sanded down. This will give the first line of tiles a lift, rather than sitting flush with the roof.
I bought a tester tin of black masonry paint and mixed it with some white emulsion to give me a suitable grey for the roof. I've painted the whole of the roof even though I am fitting tiles. This way, if there are any gaps between tiles, the MDF won't be visible. The next step will be to fit the roof tiles.