Wash the leeks and finely chop them. Cook them gently in a large knob of margarine until soft. Meanwhile peel the potatoes and slice them. Boil them for 10 minutes. Once the leeks have cooked add the milk to them and then transfer to a baking dish. Once the potatoes have boiled for 10 minutes place them on top of the leek mixture. Spread the potatoes with a layer of margarine and cook in a preheated 200 degree oven for 30 minutes or until the potatoes are golden brown.
