Preheat the oven to 200C/400F/Gas 6.
In a large bowl, mix together the two types of flour, bicarbonate of soda and salt.
Add the buttermilk and mix until a sticky dough forms.
Lightly flour a work surface and tip the dough onto it.
Gently roll and fold the dough a couple of times to bring the mixture together. Do not knead.
Shape the dough into a ball. Flatten the ball 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 is best eaten on the day of baking.