Make the pastry case. In a large bowl, whisk together the flour, sugar, and salt. Rub in the butter until the mixture resembles breadcrumbs. Add just enough ice water to bring the dough together.
Form the dough into a ball, flatten into a disk, wrap with cling film and refrigerate for at least 30 minutes.
Place the chilled dough on a lightly floured surface and flatten slightly. Using a floured rolling pin, roll the dough out until 6mm and 25–28cm in diameter. Drape the pastry over the rolling pin and place it in your pie dish. Press the bottom and sides down firmly.
Crimp the edges using the side of your forefinger and thumb. Freeze the pastry for 10 minutes.
Preheat the oven to 220°C/Gas Mark 7.
Remove the pastry case from the freezer and prick it all over with a fork. Place a sheet of baking paper in it and fill with baking beans or rice. Bake for 12 minutes. Remove from the oven and take out the baking paper and weights.
Reduce the oven temperature to 190°C/Gas Mark 5.
In a large bowl, combine the pumpkin purée, cream, eggs, both sugars, and spices. Pour the mixture into the pastry case and bake until the centre is just set, or 35–40 minutes. Let the pie cool completely before slicing.