Preheat oven to 200C/400F/Gas 6. Sift the soda flour and bicarbonate of soda into a large mixing bowl. Stir in the whole meal flour, salt and sugar until blended.
Make a well in the center and pour in almost all of the buttermilk, stirring with a round bladed knife or wooden spoon to form a loose dough.
Butter a 2-pounds loaf tin. Turn the dough into the tin, leaving the surface rough. Sprinkle with a little extra whole meal flour to give a nutty surface.
Set the tin on a baking sheet and bake for 30 minutes, then reduce the temperature to 150C/300F/Gas 2; cook for a further 30 minutes until the bread is well risen, brown and crusty on top.
The bread is cooked when a skewer inserted into the center comes out clean. Remove from oven, turn out the loaf and wrap in a clean cloth; cool completely on a wire rack.
