Put the beans in a bowl, cover with plenty of cold water and leave to soak overnight.
Drain the beans and put in a large saucepan. Cover with water and bring to the boil. Boil for 10 minutes, then drain the beans through a colander.
Put the guinea fowl in the pan and add the drained beans, onion, garlic, stock and cloves. Bring just to the boil, then reduce the heat to its lowest setting and cover with a lid. Cook very gently for 1¼ hours until the guinea fowl is very tender.
Drain the guinea fowl to a plate and leave until cool enough to handle. Flake all the meat from the bones, discarding the skin. Chop up any large pieces of meat and return all the meat to the pan.
Scoop a little of the stock into a small bowl with the anchovies and mix together so that the anchovies are blended with the stock. Discard the tough stalks from the watercress.
Add the anchovy mixture, mushrooms and tomato purée to the pan and season with salt and plenty of black pepper. Reheat gently for a few minutes and stir in the watercress just before serving.