Clean and scale the fish, split open and remove the bones. Lay the fish in a greased ovenproof dish.
Melt 2 oz (50 g) of the butter in a pan, stir in the flour and tomato purée, then add the milk, salt and pepper. Stir until the sauce boils and thickens. Pour the sauce over the fish and arrange the sliced potatoes on top; dot with the remaining butter and sprinkle with cheese. Bake in the oven at gas mark 4, 350°F (180°C), for 15 minutes. Place under a hot grill to brown just before serving.