Put the macaroni in the rice cooker. Add the stock, salt, pepper and mustard.
Put the lid on and cook for about 20 minutes on "White Rice" setting. Give it a stir after 10 minutes.
After 20 minutes, add the cheese and the cream, and give it a good stir. Add as much of the milk as necessary to make the mixture liquid, but do not add too much (you might not use all of the milk).
Put the lid on and cook it for a further 10 minutes. Stir, and check pasta is fully cooked.
If the mixture is too liquid, simply keep cooking for a few minutes, with the lid *off*, stiring all the time.