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.