Pre heat oven to 200°C/Fan180°C/GM6. Defrost chicken breasts and cut into bitesize pieces. Melt butter in a skillet pan, add carrots and leeks. Cook until softened, add flour and cook for about a minute, stirring constantly. Gradually stir in stock and bring to a simmer until thickened, add chicken, petit pois, cream, Dijon mustard, chopped parsley, thyme, salt, and pepper. Remove from heat then place into pie dish. Cut pastry squares into quarters, brush with egg wash and arrange on top of chicken mixture. Bake for around 20 minutes until top is browned and filling is bubbly. Remove from oven, and let stand 10 minutes. Sprinkle with fresh thyme leaves, and serve.