Mix sifted flour, baking powder and salt into a bowl. Add the margerine and lightly rub together until the combined ingredients resemble breadcrumbs. Add the milk into the flour mixture and lightly combine with a large spoon taking care not to overwork it or beat in any way. Add perhaps a dessertspoon extra milk if the dough does not seem quite moist enough. Drop dessertspoon sized scoops of the dough into the casserole which is gently bubbling. You will need a knife to scrape it out. Lid on the casserole and check after 10 minutes when they should have risen to twice their size. Leave for a further 10-15 mins and serve.