Put the flour, a pinch of salt and the butter in a mixing bowl. Using a table knife, cut the butter into the flour until you’re left with small cubes. Rub the butter into the flour with the tips of your fingers and thumbs until it looks like breadcrumbs, then stir in half the water. Gradually add the remaining water, mixing until the pastry comes together into a ball. Wrap in clingfilm and chill for 30 minutes.
Heat the oil in a saucepan over a high heat and add the onion, leeks and carrots. Stir well, then season and cover with a lid. Reduce the heat and cook for 10 minutes, or until softened. Then add the cream and let it bubble away until about 2 tbsp remain. Leave to cool.
Squeeze the sausage meat out of its skin and mix with the sage in a bowl. Lightly wet your hands with cold water and shape the mixture into walnut-sized balls. Set aside.
Preheat the oven to gas 6, 200°C, fan 180°C. Dust a clean surface with flour. Cut a third off the pastry and set aside. Roll out the larger piece of pastry to line a 20cm (8in) pie dish. Drape the pastry over a rolling pin and gently ease it into the dish. Use your fingers to press it into the sides of the dish. Trim any excess pastry with a sharp knife. Roll out the remaining pastry to make a pie lid.
Fill the pie with the cooled veg then layer with the sausage meat balls and slices of chicken making sure that everything is tightly packed. Brush around the edge of the pie dish with a little egg, then position the lid on top. Press the edges together, carefully pinching with your fingers to make a fluted border. Trim any excess pastry with a sharp knife. Brush all over with the egg.
Roll out any pastry trimmings and stamp (or cut) out some letters and shapes to decorate. Arrange on the pie and brush with more egg. Using a sharp knife cut a small cross in the centre of the pie to help the steam escape. Bake for 30-35 minutes, or until golden and cooked through.