Here you can see the card I used for the slates. Each slate was 10mm x 5mm. In addition I have included the small section of additional Cotswold stone wall that was attached to the side of the building. This was modelled from a section of Plaster of Paris that was carved to shape and then topped with some green foam slabs.
The final image shows work-in-progress. Notice how I apply the first layer of tiles over a backing strip (sticking out from the lower roof), this is so the angle of the roof tiles remains constant all the way up the roof and is something that many modellers neglect when building tiled roofs.