1. Preheat oven to 220°C. Heat oil in a large saucepan over medium heat. Add leek and potatoes. Cook, stirring, for 3 minutes or until leeks are tender but not coloured.
2. Sprinkle over the flour. Cook, stirring, for 1 minute. Remove from heat and gradually add stock, stirring. Return to medium heat. Cook for 3 to 5 minutes or until mixture comes to the boil. Add chicken and cheese. Season with salt and cracked black pepper.
3. Spoon chicken mixture into a 5-cup capacity, 26cm (top) ceramic pie dish. Place pastry over filling. Press edges to seal. Trim excess pastry. Cut a small cross in the centre. Bake for 15 to 20 minutes or until pastry is puffed and golden. Stand pie in dish for 5 minutes. Serve.