Soak the beans overnight in cold water. Drain, cover again with fresh water, and allow to simmer on a low heat for about 1 hour, or until the skins of the beans burst when blown upon. Drain again, saving about 1 pint of the liquid. Scald the salt pork quickly; drain pork and cut slashes in the rind with a sharp knife. Cut pork in 2 pieces; place 1 piece in bottom of a large casserole; add beans and bury remainder of pork in the beans so that the rind just shows. Mix finely chopped onions, mustard, salt, pepper, treacle and sugar with reserved bean water, and bring to a boil. Pour this mixture over the beans, to cover. If necessary, add more boiling water. Place lid on casserole and bake in a slow oven (300° to 325°-M1 to 2) for 6 hours. From time to time add more boiling water, so that beans are kept covered and moist.
One hour before serving: stir 2 to 4 tablespoons tomato ketchup into the beans; drip the hot melted bacon fat over them and cook for 1 hour in a slow oven, uncovered, to colour beans and brown pork. If desired, add sautéed onion rings, cooked sausages and frankfurters just before serving.