In a large pan, melt the margarine over medium heat. Add the mustard seeds and saute about 2 minutes. Add the whole cayenne pepper, peppercorns, onions and bell pepper and saute about 3 minutes, or until the onions are tender. Add the turmeric, coriander, salt and ground cayenne. Stir well to blend. Add the potatoes, water, and lemon juice and stir well. Reduce the heat to low and simmer, covered for 3-5 minutes or until the sauce thickens. Remove the whole cayenne pepper before serving.