Preheat oven to 180°C fan forced (200°C normal). Cook the potato in a pot of lightly salted, boiling water until tender. Then drain well, roughly mash and set aside. At the same time, heat a little oil and the butter in a heavy-bottomed pan and gently sauté the leek until also tender, stirring now and again. Then add the mashed potato, peas and grated cheese with a little salt. Stir until well combined. Beat the milk with the egg. Cut out two large pastry rounds and place one in a lightly oiled, large baking tray. Brush the edges with egg wash and spoon the mixture on top, leaving an edge. Then place the other pastry round on top, crimp the edges with a fork and generously brush all over with egg wash. Cook in the oven for about 15-20 mins. Serve with a salad on the side.