At this point pre-heat the oven to 190°C / 170°C fan / gas mark 5 / 375°F.
Cut two large pieces of baking parchment, 2½ inches wider than the size you need to roll the pastry out to and dust with a little tapioca flour
Place the larger of the chilled pastry discs between the parchment sheets, dusting the top of the pastry with a little tapioca flour.
Using a long rolling pin, roll out the pastry into a round until it is about 3-4mm thick and a couple of inches larger than your pie dish.
Peel away the baking parchment and use the rolling pin to lift the pastry and drape it over the pie dish.
Fit the pastry into the pie dish, patching up any cracking with excess pastry. Place the pie dish in the fridge whilst you roll out the second pastry disc in exactly the same way.
Bring your gooseberry filling out of the fridge after you have rolled out the second disc. Stir the tapioca starch into the gooseberry filling until totally absorbed.
Spoon the gooseberry filling into the pie dish in an even layer.
Dampen the edges of the pastry with a little water which will help seal the pastry lid on top.
Drape the second disc of pastry carefully over the top of the pie. Press the edges of the pastry together then cut away the excess pastry to the edges of the pie dish. Crimp the edges together using your hands or the back of a fork.
Make four slits in the centre of the pie lid using a sharp knife.
Brush the egg wash all over the top of the pie, making sure it doesn’t gather to much in the crevices. The sprinkle over the 2 teaspoons caster sugar.
Bake the pie in the middle shelf of the oven for 45 minutes until the pie is beautifully golden.
Remove from the oven and allow the pie to cool in the tin.