Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the pork a few pieces at a time until all the pieces are in the pan, then fry over a high heat until lightly browned. Lift out of pan with a slotted spoon and transfer to the slow cooker pot.
Add the white leek slices to the pan and fry for 2–3 minutes or until softened. Stir in the flour, then gradually mix in the cider and stock. Add the carrot, apple, sage and some salt and pepper. Bring to the boil, stirring. Pour the mixture into the slow cooker pot, cover with the lid and cook on low for 8–10 hours or until the pork is tender.
Make the dumplings. Put the flour, suet, herbs and a little salt and pepper into a bowl, mix together, then gradually stir in enough water to make a soft but not sticky dough. Cut into 12 pieces and roll into balls with floured hands. Stir the green leek slices into the pork casserole and arrange the dumplings on the top. Cover and cook, still on low, for 1 hour until they are well risen. Spoon into shallow bowls to serve.