Pre-heat oven to Gas Mark 6, 200ºC (400ºF). Sieve the dry ingredients into a large bowl. Rub in the margarine. Pour in the egg and buttermilk mixture and stir well. Turn out onto a floured board and knead lightly for a minute or two. Shape into a round and flatten slightly.
Place on a baking sheet. Mark with a deep cross and bake in the oven for approximately 30-40 minutes.
Cool on a wire rack.