Rinse beans in cold water then place in a large saucepan of boiling water. Return to the boil and cook for 2 minutes.
Remove from heat, cover and leave to swell and soften for 1 hour. Drain.
Place tomatoes, treacle, onion, dry mustard, Worcestershire Saauce and salt in a large casserole. Stir in prepared beans. Cover and place in a slow oven for 4 - 6 hours. Very slow baking allows the full flavour to develop.