Tip the flours, baking soda and salt into a large bowl. Then use your fingertips to rub the butter into the flour until evenly dispersed.
Stir in the sugar.
Make a well in the centre of the flour and stir in the buttermilk until you get a soft dough.
Turn out onto a lightly floured surface and lightly knead for 1 min until smooth, then bring together to form a flattened round about 5cm thick and 15cm across.
Place on a lightly dusted baking sheet. Dust with a little plain flour if you like. Use a large sharp knife to cut 2 slashes across the bread to make an ‘x’. Bake in the oven for 35 – 40mins until well-risen and browned. Place on a wire rack and leave to cool.