Trim the leeks, discarding most of the dark green part. Slice thickly and wash under cold running water until completely clean. Cook in boiling salted water for 8-10 minutes or until almost tender. Drain thoroughly, reserving the stock.

At the same time, cook the potatoes in boiling salted water for about 20 - 25 minutes or until tender. Drain.

While the vegetables are cooking, make the sauce: melt 25 g (1 oz) margarine gently in a small saucepan, sprinkle in the flour and stir over low heat for 2 minutes until straw-coloured. Remove from the heat and gradually stir in all but 2 tablespoons of the milk, then return to the heat and simmer, stirring, until thick and smooth. Measure out 150 ml (1/4 pint) of the leek stock; stir gradually into white sauce. Bring back to the boil, stirring constantly,

then add the nutmeg and salt and pepper to taste. Remove the pan from the heat.

Heat the oven to 190°C (375°F) Gas 5.

Slice one-third of the potatoes, stir them gently into the leeks and turn into the base of a greased 1 L/2 pint ovenproof dish. Pour on the sauce; carefully turn vegetables with a fork, to coat thoroughly.

Add remaining margarine and milk to the rest of the potatoes, season with pepper, then mash them until smooth. Beat in half the cheese with a wooden spoon. Taste and adjust seasoning.

Spread the mashed potato over the vegetables, then sprinkle on the rest of the cheese. Stand the dish on a baking sheet and bake in the oven for 20 - 25 minutes, or until the topping is golden brown. Serve hot.