Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a frying pan, add the chicken joints and fry over a high heat until browned on all sides. Transfer to the slow cooker pot with a slotted spoon.
Add the bacon and white sliced leeks to the frying pan and fry, stirring, for 5 minutes or until just beginning to turn golden. Stir in the flour, then gradually mix in the stock, mustard and a little salt and pepper. Bring to the boil. Pour into the slow cooker pot, cover with the lid and cook on low for 8–10 hours.
Add the green sliced leeks and stir into the sauce, then replace the lid and cook, still on low, for 15 minutes or until the green leeks are just softened. Spoon into shallow serving bowls and serve with mashed potato.