Preheat the oven to 200C / Gas 6. Sift the flours, bicarb and salt into a bowl, tipping in any bran left in the sieve. Stir in the raisins, mixed peel and lemon zest.
Make a well in the centre and pour in the buttermilk. With a wooden spoon, gradually stir the flour into the buttermilk to form a soft dough. Bring the dough together with your hands, then turn it out onto a lightly floured work surface. Knead lightly and briefly until it forms a smooth ball.
Place the dough on a greased baking tray and flatten it slightly to make a domed round loaf about 20cm in diameter. Using a sharp knife, cut a deep cross in the top of the loaf, cutting about halfway down into the dough. Sprinkle a little extra flour over the top.
Bake until well risen and browned, and the bread sounds hollow when tapped on the base, about 30 minutes.
Transfer to a wire rack and leave to cool completely.