While the pasta is cooking, make the cheese sauce. Melt the butter in a saucepan and stir in the flour, till it starts to bubble slightly. Add the bay leaf, turn up and heat and pour in a quarter of the milk, stirring all the time. As the sauce start to thicken, add more milk, stirring all the while, repeating until milk is all used up. Then stir in the grated cheddar cheese and mustard powder.