Preheat the slow cooker if necessary; see the manufacturer's instructions. If the turkey drumstick does not fit into the slow cooker pot sever the knuckle end with a large heavy knife, hitting it with a rolling pin.
Heat the oil in a large frying pan, add the drumstick, bacon and sausage pieces and fry, turning until browned all over. Transfer to the slow cooker pot. Add the onion and white leeks slices to the pan and fry until softened. Stir in the flour, then mix in the stock. Add the herbs, salt and pepper and bring to the boil.
Add the carrots, celery and cranberries to the pot and pour over the hot onion mixture. Cover with the lid and cook on high for 5–6 hours or until the turkey is almost falling off the bone. Lift the turkey out of the slow cooker pot. Remove and discard the skin, then cut the meat into pieces, discarding the bones and tendons. Return meat to the pot with the reserved green leek slices.
Make the dumplings. Mix the flour, suet, parsley and salt and pepper in a bowl. Stir in enough water to make a soft dough. Knead, then shape into 12 small balls. Arrange over the turkey, replace the lid and cook, still on high, for 30–45 minutes or until the dumplings are cooked through. Spoon into shallow bowls to serve.