Melt the butter in a sauce pan.

Stir in the flour and cook for 1-2 minutes.

Take the pan off the heat and gradually stir in the milk to get a smooth sauce. Return to the heat and, stirring all the time, bring to the boil.

Simmer gently for 8-10 minutes and season with salt.

Stir in the cheese and allow to melt. don't reboil the sauce as it will become stringy.