1. Preheat the oven to 200C/400F/gas mark 6. Grease a 23cm/9 inch cake tin and line base with greaseproof paper.
2. Sift the dry ingredients into a bowl. Add the remaining ingredients and beat for 1-2 minutes until smooth. Spread the mixture evenly in the cake tin.
3. For the topping, mix the dry ingredients together and rub in the margarine to a crumbly texture. Sprinkle over the cake mixture in the tin.
4. Cook for about 25-30 minutes until risen.