Preheat the oven to 220 C / Gas 7. Place the cubed potato in a saucepan with enough water to cover. Bring to the boil, and cook until tender, about 5 minutes. Drain, mash and set aside.
Meanwhile, add the minced beef and pork to a large frying pan, breaking it apart with a wooden spoon. Add the garlic, onion and water. Season with mustard powder, sage, thyme, cloves, pepper and salt. Cook over medium heat till the meat is evenly browned. Remove from the heat, and mix in the mashed potato.
Place one of the sheets of pastry in a 23cm (9 in) pie dish. Fill with the meat mixture, then top with the other pastry sheet. Prick the top a few times with a knife to vent steam. Crimp around the edges using the tines of the fork, or pinch with your fingers, and remove any excess pastry.
Bake for 25 minutes in the preheated oven, or until the pastry has browned.