Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter in a frying pan, add the gammon and leeks and fry, stirring, for 4–5 minutes or until the leeks have just softened. Season with pepper only. Leave to cool slightly.
Put the flour, ½ teaspoon salt, a large pinch of pepper, the suet and mustard powder in a bowl and mix well. Gradually stir in enough water to make a soft but not sticky dough. Knead lightly, then roll out on a large piece of floured nonstick baking paper to a rectangle 23 x 30 cm (9 x 12 inches). Turn the paper so that the shorter edges are facing you.
Spoon the gammon mixture over the pastry, leaving 2 cm (¾ inch) around the edges. Roll up, starting at the shorter edge, using the paper to help. Wrap in the paper, then in a sheet of foil. Twist the ends together tightly, leaving some space for the pudding to rise.
Transfer the pudding to the slow cooker pot and raise off the base slightly by standing it on 2 ramekin dishes. Pour boiling water into the pot to come a little up the sides of the pudding, being careful that the water cannot seep through any joins. Cover with the lid and cook on high for 4–5 hours or until the pudding is well risen.
Just before serving, melt the butter for the sauce in a saucepan. Stir in the flour, then gradually mix in the milk and bring to the boil, stirring until smooth. Cook for 1–2 minutes, then stir in the parsley and season. Lift the pudding out of the slow cooker pot, unwrap and cut into slices. Arrange on plates and spoon over a little sauce. Serve with sugar snap peas.