Preheat oven to 160 degrees C. Grease a 12x23cm loaf tin.
Combine flour, baking powder, baking soda, salt and sugar in a large bowl. Blend egg and buttermilk together in a separate bowl, and add all at once to the flour mixture. Mix just until moistened. Stir in butter. Pour batter into loaf tin.
Bake for about 1 hour, or until a skewer inserted into the centre comes out clean. Cool on a rack. Wrap in a clean tea towel for several hours, or overnight, for best flavour.