Preheat the oven to 180 C / Gas 4. Grease a 22x18cm baking tin and line with baking parchment.
Place the flour into a mixing bowl then cut in the margarine and mix with your fingers to form breadcrumbs. Stir in the sugar and the sultanas and then add just enough milk to make a soft consistency. Transfer to the prepared baking tin and press into the tin with the back of a spoon.
Bake in the preheated oven for 20 to 30 minutes or until golden brown. Remove from the oven and mark into squares. Allow to cool in the tin before removing, slicing and serving.