I have, at last, been weaned off using proprietary roofing slates thanks to the method Iain Robinson describes in the comment at the end of the post in his blog. In case this helps someone else, I used 3x3mm squared graph paper from the web, printed on 100 gsm paper, cut into 6x3 rectangles, stuck down with PVA, painted with what I guessed was an overall slatey sort of colour, then picked out individual slates in different shades. This first attempt is not perfect but much more pleasing - and satisfying. It makes me think I may have to revisit the roofs of earlier models now though. (I bet I won't. Too lazy). The ridge tiles are strips of paper laid over a piece of string glued to the ridge, with thinner strips applied for the joins. I copied that idea too. I copy everything.
Here's a nice roofing example too on the lnr models blog.