Put the beans in a bowl, cover with cold water and leave to soak overnight.
Drain the beans and put in a flameproof casserole. Cover with water and bring to the boil. Reduce the heat and simmer gently for 15–20 minutes or until the beans have softened slightly. Test by removing a few on a fork and squeezing them gently – they should give a little. Drain the beans.
Wipe out the dish and melt the butter. Add the bacon and pork and fry gently for 10 minutes until beginning to brown. Add the onion and cook for a further 5 minutes.
Stir in the drained beans, thyme or rosemary and tomatoes. Add enough water to just cover the ingredients and bring to the boil. Cover with a lid and transfer to a preheated oven, 150°C (300°F), Gas Mark 2. Cook for about 1 hour or until the beans are very tender.
Mix together the treacle, tomato purée, mustard, Worcestershire sauce and seasoning. Stir into the beans and return to the oven for a further 30 minutes.