Preheat the oven to 165*C/325&F/ gas mark 3. Drain all the beans and rinse them in cold water. Drain them again and then put them into a 9 by 13 inch glass baking dish, of small roasting tin. Mix the ketchup, molasses, salt, black pepper, mustard powder, tomato sauce, brown sugar and Worcestershire sauce in a small saucepan. Bring to the boil over medium heat, stirring to melt the sugar. Remove from the heat. Stir in the onions. Pour half of this mixture over the beans in the dish. Top the beans with the slices of bacon and then spoon the remainder of the sauce over top. Cover tightly with a sheet of aluminium foil. Bake for 3 hours until the beans are tender and delicious. Serve hot with your favourite accompaniments.