Measure 3 pints (1.7 litres) water into a large saucepan, add the beans, bring to the boil and boil gently for about 2-3 minutes; then remove the pan from the heat and leave on one side for about 1 hour, or until the water has cooled.
Now return the beans to the heat and simmer uncovered until the bean skins burst when you lift them out of the water (which will take around 45 minutes). Drain the beans next, reserving the liquor, then measure the liquor and make it up to 1 pint (570 ml) with water if necessary. Transfer the beans to a casserole and pre-heat the oven to gas mark ½, 250°F (120°C).
At this stage, blend the mustard powder with a little of the measured bean liquor, followed by the black treacle, sugar, tomato purée and crushed garlic, and pour this mixture over the beans, along with the measured pint (570ml) of liquid, some seasoning, the sliced onion and a bay leaf. Now cut slashes across the pork (approximately ½ inch/1 cm apart) and bury the meat in the beans until only the rind is showing. Then cover the casserole tightly and bake very slowly for about 6 hours.
During the last hour of cooking, take the lid off the casserole to allow the rind on the pork to crisp a little. Also, keep stirring the beans during this last hour and, if they show signs of getting too dry, add just a spot more water.
Serve very hot.
You will also need a large saucepan and a lidded casserole with a capacity of 6 pints (3.5 litres).