Preheat the oven to 230°C/Fan 210°C/Gas 8. Sift the flours into a large bowl, tipping in any bran left in the sieve. Stir in the salt, then rub in the lard. Stir in the bicarbonate of soda, cream of tartar and sugar.
Make a well in the centre and pour in the buttermilk. Quickly mix together with a wooden spoon and tip onto a lightly floured work surface.
Shape very lightly into a ball and put on a lightly greased baking tray. Flatten slightly then, using the handle of a wooden spoon, mark a deep cross almost two-thirds of the way through. Bake for 30 minutes. Cool on a wire rack. Best eaten on day of baking.