Cook the potatoes in salted boiling water. Cut the green parts off the leeks, wash the white parts well and cut into ½ inch (1 cm) rounds. Melt the butter in a casserole, toss in the leeks, season with salt and freshly ground pepper, cover and cook for 20 minutes on a very low heat. Make the cheese sauce (see Mornay Sauce recipe) but add a crushed clove of garlic; it should be a light coating consistency. When the potatoes are cooked, peel and cut into ½ inch (1 cm) cubes and mix gently with the leeks and sauce. Turn into a pie dish. Sprinkle with grated Cheddar cheese. This pie may be prepared ahead and reheated later in a moderate oven, 180°C/350°F/regulo 4, until golden and bubbly on top, 20 minutes approx.