Preheat the oven to 400F/200C/Gas 6.
Tip the flours, salt and bicarbonate of soda into a large mixing bowl and stir.
Make a well in the centre and pour in the buttermilk, mixing quickly with a large fork to form a soft dough. (Depending upon the absorbency of the flour, you may need to add a little milk if the dough seems too stiff but it should not be too wet or sticky.)
Turn onto a lightly floured surface and knead briefly.
Form into a round and flatten the dough slightly before placing on a lightly floured baking sheet.
Cut a cross on the top and bake for about 30 minutes or until the loaf sounds hollow when tapped. Cool on a wire rack.