Use a baking tray; grease tray. Preheat oven to 200 degrees C. Sift flours, bicarb soda and salt into a bowl, tipping in any bran left in the sieve.
Make a well in the centre and pour in the buttermilk. Using a wooden spoon, gradually stir flour into the buttermilk to form a soft dough. Bring dough together with your hands, then turn it out onto a lightly floured work surface. Knead briefly to a smooth dough. Shape into a ball.
Place dough on baking tray and flatten slightly to make a round, domed loaf about 19 cm in diameter. Using a sharp knife, make a deep cross in the top, cutting half-way down into the dough. Sprinkle extra wholemeal flour over the top.
Bake for about 30 minutes or until well risen and browned and the bread sounds hollow when tapped on the base. (If it still seems to be moist and heavy, bake for a further 3–5 minutes and test again.)
Transfer loaf to a wire rack and leave to cool completely. Serve on the day it is baked, as it quickly becomes stale. Or, you can toast it the following day.