Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the chicken a few pieces at a time until all the meat is in the pan, then add the onion and bacon and fry, stirring, until the chicken is golden.
Stir in the flour, then gradually mix in the stock. Add the sage sprigs and a little salt and pepper and bring to the boil, stirring. Add the black pudding, if using, carrots and swede to the slow cooker pot.
Pour over the hot chicken mixture, then arrange the potatoes overlapping on the top and press below the surface of the liquid. Sprinkle with a little extra salt and pepper, then cover with the lid and cook on high for 4–5 hours or until the potatoes are tender and the chicken is cooked through.
Lift the pot out of the housing using oven gloves, dot the potatoes with butter and brown under a hot grill. Spoon into shallow dishes to serve.