To prevent the gammon from curling up while it's cooking, just snip the fat at about 1 inch (2.5 cm) intervals all round.
Then combine all the ingredients for the sauce – the honey, mustard and a few twists of pepper – in a bowl. Now put a small roasting tin (or gratin dish) containing the butter under the grill and heat the butter until it starts to sizzle.
Then remove the dish from the heat and pour in the honey-mustard mixture, stirring to combine it with the butter. Next take the gammon slice and drag it through the mixture and lay it – sauced side up – in the tin or dish and grill about 6 inches (15 cm) from the heat for 5 minutes on each side.