Remove the fish gently from the milk and set aside. In a small bowl, add the cornflour and mix with a little drop of water to form a thin liquid. Return the milk to a low heat and whisk the cornflour liquid in.
Continue to whisk gently. Add the mustard powder and whole grain mustard until the sauce thickens. If you prefer a thicker or thinner sauce then add more or less cornflour - just change the Points and calories etc accordingly. Season the sauce with salt and pepper.