Melt the butter in a pan, then add the flour. Cook for 1-2 minutes, stirring with a wooden spoon. This is the roux which is the basis of the sauce.
Add about a fifth of the milk to the pan, whisk well and it will become incorporated with the roux.
Add another lot of milk, and again whisk it in well.
Continue, adding the milk in about 4 more lots, and whisking after each addition.
Set the pan to cook gently for around ten minutes, it should be at a simmer. Stir occasionally. About two minutes from the end the sauce will thicken, make sure you stir it well and whisk during this time.
Once thickened your sauce is ready to use. Note it will get very thick, and you want it to get very thick before adding it into the slow cooker.
To make the mac and cheese:
Add the dry macaroni pasta and then the cheese in the slow cooker pot.
Pour on the cheese sauce, and add the cold water.
Add the herbs and spices, and season well with salt and pepper. Taste the sauce and make sure it is seasoned to your taste.
Stir well, put on the lid and cook for 1 hour 45 minutes to 2 hours. Stir occasionally so it doesn't get stuck to the base of the slow cooker pot. Serve once ready.Note that timings vary depending on your own slow cooker, so check the dish occasionally and if the pasta is still hard, cook for longer, or if it is soft and ready to eat earlier, take it out earlier.