Pre-heat the 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. Allow to cool on a wire rack.