Rinse the beans and put them into a large saucepan. Cover with cold water and bring to the boil. Skim the surface of the scum with a slotted spoon. Add 2 whole garlic cloves and one of the sage sprigs. Cook for 1-1½ hours or until the beans are tender. Season with salt and pepper in the last half hour of the cooking time.
In a separate large saucepan, heat the olive oil. Prick the sausages in a couple of places with a fork and add them to the pan. Fry on a medium-high heat to brown on all sides. Add the remaining garlic clove and the sage, and as soon as it begins to sizzle, add the tomatoes. The sausages should provide enough seasoning; if not add a little salt and pepper. Simmer for about 15 minutes or until the tomatoes have melted into a sauce.
Drain the cooked beans, reserving a little of the cooking liquid and add the beans to the sausage pot. Check the seasoning and add salt and pepper, if necessary. Simmer for 10 minutes more, adding about