Preheat oven to 190°C/375°F/Gas Mark 5. Grease baking tray with the butter and dust with flour.
Sieve the flours, Baking Powder, Bicarbonate of Soda, sugar and salt in a bowl and add any bran remaining in the sieve.
In a jug, beat together the egg and yogurt and pour the mixture into the dry ingredients. Mix well, bringing the ingredients together to form a sticky dough.
Knead the dough on a lightly floured work surface for a few minutes until smooth, then shape into a large round about 5cm (2 inches) deep.
Transfer the dough to the baking tray. Mark a cross shape one the top with a sharp knife.
Bake in the oven for about 40 minutes, or until the soda bread is golden brown all over. Transfer the loaf to a wire rack and leave to cool completely. Cut into slices to serve.