Put the milk, all the cheeses, mustard powder, a generous pinch of salt and freshly ground black pepper into a slow cooker. Stir well, then add the macaroni and mix again. Cook on low for 1 1/2- 2 1/2 hours until the pasta is cooked and the sauce is creamy. Stir in a splash of milk if the mixture is a little stiff. Cook bacon, then add before serving (you can use ready-cooked bacon if you prefer).