Season the pork with a little salt and pepper. Melt the butter in a frying pan on the hob and lightly brown the pork on all sides. Transfer to a casserole dish. Add the bacon and leeks to the same frying pan and fry gently for 5 minutes. Stir in the plain flour and mix to a paste. Gradually blend in the milk and whisk until smooth. Bring to the boil and stir in the thyme and chilli powder. Tip the contents of the pan into the casserole.
Cover the dish with foil and place on the lower rack of the oven. Set the temperature to 180°C (350°F) and cook for 25 minutes or until tender.
Meanwhile towards the end of the cooking time, make the dumplings. Put the 2 flours, baking powder, breadcrumbs, chives and poppy seeds in a bowl. In a separate small bowl, beat the egg with the milk and then add to the flour mixture, mixing with a round-bladed knife to form a dough and adding a dash more milk if the dough feels dry. Carefully drop spoonfuls of the dough over the pork, cover again with foil and cook for a further 10 minutes. Remove the foil and cook for a further 5 minutes to brown the surface slightly.