I've now cut and glued sandpaper to the flat part of the roof to give the appearance of a tarred surface, similar to that on the sides of the dormer windows.
Now it is ready for painting into grey, but I will need a darker grey for the 'lead' flashing.
I found tubes of red and brown acrylic paint and mixed a bit until I had a deep terracotta for the chimney pots.
I'm trying to find a suitable paint in our garage for the walls now. I've decided to paint the top half with a white emulsion mixed with sand. The bottom half of the walls will have 'bricks' made from stipple painted sandpaper. I'll need a sandy/grey colour for the base to look like mortar.
Before all of this, I'm going to tile the roof of the shop canopy, so that all the grey paintwork can be carried out in one go.
I've had to insert one of the lights as the wire groove is under the flat roof. I've covered the groove with card, so that the glue from the roof doesn't stop me from pulling the light to and fro to fit it properly once the room is decorated.