Bring a large, covered saucepan of unsalted water to the boil. Drain all of the beans and lentils and add them to the pan. Re-cover the pan, return to the boil and boil vigorously for 10 minutes.
Meanwhile, put the potatoes, garlic, celery, onion, celery seeds and marjoram in the slow cooker.
At the end of the boiling time, drain and rinse the beans, then transfer to the cooker.