(Touch wood) the probelem seems to have resolved itself. The main thing I did was increase colour depth from "thousands" to "millions". It seems to make no difference whatsoever to the image as displayed but, funnily enough, CM:BB hasn't frozen once since I did it. Coincidence? Who knows but whatever I did seems to have worked.
I have noticed a couple of funnies in the scenario editor, but I'll write them up as a separate thread.