Cook the leeks and broccoli seperately in a little boiling water for about five minutes. Drain well. Arrange the broccoli and leeks in an ovenproof dish.
Melt 40 g of the margarine in a frying pan over a moderate heat. Add the breadcrumbs and fry until golden. Stir in the yoghurt, season to taste. Mix in half the cheese.
Pour the cheese sauce over the vegetables. Sprinkle with the remaining cheese and dot with the margarine. Cook in the oven for 15 - 20 minutes, then brown under a hot grill if wished. Serve immediately, sprinkled with paprika.