Melt the butter in a saucepan and mix in the flour. Stir in the milk and simmer gently until smooth and thick. Add the cheese, salt and pepper. When well mixed, pour the mixture on to the well-beaten egg yolks. Whisk the egg whites and stir them lightly into the mixture. Grease a fondue dish or tin which is large enough for the mixture to ½ fill it. Pour in the mixture and bake in a fairly hot oven (375° F., Gas 5) for about 20 min. This dish should be served the moment it is ready.