In a medium saucepan over medium heat, melt butter. Whisk in the flour and cook for a couple minutes without browning. Add the milk and reduce heat to medium-low. Simmer gently until the flavour of the flour has dissipated. Add all the cheeses and cook until melted. Add cooked macaroni and remove from heat. If desired, season with salt and pepper. Transfer to a casserole dish and cover with foil. To rewarm, simply place the whole casserole dish on the grill or enjoy cold as part of a pasta salad.