Add the tomatoes and toss to distribute through the beans. Add chicken stock, salt and a good twist of cracked pepper. If necessary, top with a little water to only just cover the beans. With the lid on, simmer for 2 to 2½ hours checking the liquid level and gently stirring every now and then. Towards the end of cooking, you may need to remove the lid and reduce the liquid a little to create a thin sauce and not be too soupy.