Place the lamb on top. Cut the remaining garlic and lemon in half laterally, squeeze the lemon briefly over the potatoes, and tuck the shells and the garlic in around the joint along with the bay leaf. Pour 200ml water into the dish. If using a casserole dish, tuck a damp piece of greaseproof on top and cover, if using a roasting dish, sprinkle the overhanging paper with water and fold over and tuck in to form a sealed package. Bake for 4-5 hours until very tender.