Preheat the oven to 200C/400F/Gas 6, or use the top oven of an AGA.
In a large bowl, sieve and mix together the two types of flour, baking powder, bicarbonate of soda and salt.
Rub in the butter/margarine.
Add the buttermilk or mixed milk and yoghurt, and mix gently to a sticky dough.
Put the dough onto a cold lightly flour dusted work surface.
Do not knead, just gently roll and fold and shape the dough into a round flat ball, then flatten gently with your hand. Score the dough with a deep cross dividing it into quarters. Dust the bread with flour.
Place onto a baking tray lined with baking parchment and bake for 30 minutes at 200C/400F/Gas 6. The loaf should be golden-brown.
Leave to cool on a wire rack. This loaf is best eaten on the day of baking!