Preheat the oven to 180°C / 350°F / Gas mark 4.
Grease a casserole dish or pie dish and set aside.
In a food processor, blitz the butter and flour together.
Add 2 tablespoons of 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 in a frying pan, stirring.
Sprinkle in the flour and cook, stirring, for 2 minutes.
Take off the heat and stir in the milk.
Season with salt and pepper, add the mustard and bring to the boil, stirring.
Remove the sausages from the grill, slice them into pieces and place in the prepared baking dish.
Stir the parsley into the leek sauce and pour over the sausages.
Roll out the pastry and cut out a shape 2.5cm larger than the dish.
Brush the rim of the dish with water and lay the pastry on top.
Press to seal and crimp edges.
Bake for 25 minutes.
Allow to cool a little before serving.