Preheat the oven to 180˚C fan / 200˚C / 260˚F convection / 400˚F.
To make the sauce, melt the butter or margarine in a medium saucepan over a medium heat.
Add the flour and stir well to combine, cook for 2-3 minutes.
Add the water / stock a little at a time, using a balloon whisk to beat out any lumps. When all the water / stock has been added, add the milk in two goes, stirring or whisking in between.
Stir in the wholegrain mustard and Worcestershire sauce (if using) and season with salt and pepper. If you are using bought stock you won’t need as much salt.
Heat gently for 5 minutes, stirring all the time, until the sauce has thickened slightly - you should see ribbons on the surface following the balloon whisk.
Stir in the shredded cooked chicken and sweetcorn.
Pour the chicken and sauce in to a 23cm / 9 inch pie dish then lay the puff pastry across the top.
Trim any overhanging edges, using these to decorate the surface.
Using a sharp knife, make a couple of small holes in the pastry to allow to steam to escape.
Brush with a little milk if your like (I use the milk at the bottom of the jug / bowl I measured it out in - there’s enough to lightly coat the pie top).
Place in the oven and bake for 25-30 minutes until the top is golden brown and the pie is piping hot.
Leave to cool for 5 minutes before serving.
I do not recommend you reheat the pie as the chicken has already been reheated.