Put the still wet leek in a deep saucepan over a low heat with the butter. Cover and leave to cook, stirring occasionally, for 10-15 minutes until softened. Don’t let the leeks brown.
Stir the flour into the leeks and cook for a couple of minutes then pour in the double cream. Add the cheese and the mustards, plus a little salt and black pepper.
Heat a grill and toast the crumpets. Spoon the leek and cheese mixture over and return to the grill. Cook until the surface is patchily golden.