Slightly warm the milk in a saucepan over a low heat and, in a separate pan, melt the butter. Add the flour to the butter and cook over a medium heat, stirring continuously, for about 3 minutes. Don't allow the flour to brown. Slowly add the warmed milk, beating the mixture with a hand whisk (or fork) as you do so. Continue to whisk the mixture until the sauce thickens, then cook gently for about 2-3 minutes over a low heat stirring occasionally until it reaches a rich and creamy consistency.
Crumble the cheese between your fingers (or grate it beforehand if you prefer) and add to the sauce. Continue to stir over a low heat until all the cheese has melted. If it is too thick, you can add a little more milk. Season with salt and pepper to taste.