Cook the beans in boiling water for 40 minutes, then drain well.
Meanwhile, heat the butter in a flameproof casserole dish, then cook the sausages until browned and the fat runs. Remove and drain on kitchen paper. Stir the turkey into the casserole dish and cook until lightly browned all over, then transfer to a bowl using a slotted spoon.
Stir the leeks and carrots into the casserole dish and brown lightly. Add the tomatoes and tomato purée, and simmer gently for about 5 minutes.
Chop the sausages and return to the casserole dish with the beans, turkey, bouquet garni, stock and seasoning. Cover and cook gently for about 1 1⁄4 hours until the beans are tender and there is very little liquid. Serve hot.