Thank you all, it's starting to come together. Urban rubble has always been a challenge for me; at the moment I'm trying to get a base layer of scenic scatter and shredded cork chunks. Once painted hopefully I can apply some larger pieces of debris to tie it together, but at 1/72 scale it's very difficult to get things that look like individual bricks.
Paul, the molds I use are a product called 'Blue Stuff'. They come in sticks, and you soften them with boiling water to make a mold. Then I use milliput for the actual casting. Very handy for this scale, where decent accessories are hard to come by!