Melt the butter in a small pan then stir in the Mix; remove from the heat.
Mix the milk and ale or cider together and stir into the mixture in the pan. Return to the heat and bring to the boil, stirring all the time to make a thick, smooth sauce.
Allow to bubble for a couple of minutes then remove from the heat before adding the cheese. Stir well until melted. Mix in the mustard with a couple of splashes of Worcestershire sauce and season with freshly ground black pepper.
Lightly toast the bread on one side, then spread the cheese mixture over. Grill until golden and bubbling.