Just a postscript really, to all the loafposts. I tried it again today, using 2 cups of strong plain flour and 1 cup of coarse wholemeal.
Due to unforeseen circumstances the first rising was nearly 24 hrs, but the dough did not seem to come to any harm. The second rising was 2 hrs, but the dough was quite cold to start with and I think I should have given it longer.
But no harm done, and a very good loaf emerged from the casserole dish. The crust is not so thin as with the white flour, but crunchy and cracking even so.
Definitely the way forward.