Preheat the oven to 450° F. Mix the flours in a large wide bowl, add the salt and sieved baking soda. Lift the flour up with your fingers to distribute the salt and baking soda.
Make a well in the center and pour in all the buttermilk. With your fingers stiff and outstretched, stir in a circular movement from the center to the outside of the bowl in ever increasing concentric circles. When you reach the outside of the bowl, seconds later the dough should be made.
Sprinkle a little flour on the worktop. Turn the dough out onto the floured worktop. (Fill the bowl with cold water so it will be easy to wash later.)
Sprinkle a little flour on your hands. Gently tidy the dough around the edges and transfer to oven tray. Tuck the edges underneath with your hand; gently pat the dough with your fingers into a loaf about 1 1/2-inch thick. Now wash and dry your hands.
Cut a deep cross into the bread (this is called ‘blessing the bread’ and then prick it in the center of the four sections to let the fairies out of the bread).
Bake in the preheated oven for 15 minutes then turn the oven down to 400°F for a further 15 minutes. Turn the bread upside down and cook for a further 5 to 10 minutes until cooked (the bottom should sound hollow when tapped). Cool on a wire rack.