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Preheat the oven to fan 180C/ conventional 200C/gas 6. Whizz butter and flour in a food processor. Add 2 tbsp water and pulse to a ball.Wrap the dough in cling film and chill.
Heat the grill to high, brown the sausages for 5 minutes. Fry the leeks in the butter for 2- 3 minutes. Sprinkle in flour and cook, stirring, for 2 minutes. Take off heat and stir in the milk. Season, add the mustard and bring to the boil, stirring.
Cut each sausage in five at an angle and pile into a 1.2 litre dish. Stir the parsley into the leek sauce and pour over.
Roll out the pastry and cut out a shape 2.5cm larger than the dish.Re-roll trimmings to make a 2.5cm strip. Brush the rim of the dish with water and lay the strip on top. Lift the rest on top, press to seal and crimp edges. Bake for 25 minutes.