Roll out the reserved piece of pastry on a floured sheet of baking parchment to a 27.5cm (11inch) circle. Put on top, sealing edges, by pressing well together. Brush with the rest of the beaten egg. Pierce a couple of holes in the centre for steam to escape. Lightly score the top of the pastry, if you like to make a diamond pattern. Bake for 40 minutes until golden.