In a large pan over high heat, combine the beans with water to cover and bring to the boil for 10 minutes. Remove from heat but let sit for 1 hour. Drain beans and place them in a slow cooker. Add the ham hocks, onion, brown sugar, salt and water.
Mix well, cover and cook on high setting for 4 to 5 hours, stirring occasionally. During the final hour of cooking, add the tomato ketchup and puree, remove the ham from the hocks and discard the hocks. Mix well and serve.
Instead of ham hocks, use chopped streaky bacon or pancetta. You can also adjust the sugar to taste; start with less if you are unsure, then taste after adding the tomato ketchup and puree. You can add more sugar at that point, if liked.