To make the cheese sauce, melt the margarine/butter in a saucepan. Stir in the flour and cook over a medium heat for 1 minute. Turn down the heat and add only as much milk as the mixture can absorb to make a slightly smoother paste. Work the paste into a smooth and glossy consistency before adding a little more milk. Repeat until the paste has a creamy consistency. Gradually stir in the remaining milk and turn up the heat. Bring to the boil and simmer, stirring constantly, until the sauce thickens. Stir in the cheddar cheese and English mustard. Season to taste with salt and pepper.