Combine the flour and salt. Add the butter and use a dough scraper to cut in the butter until the pieces are the size of small peas. Sprinkle 1 tablespoon of the water over part of the mixture, then gently toss with a fork. Push moistened portion to the side of the bowl. Repeat, using 1 tablespoon of water at a time, until all is moistened. Divide the dough in half. Form each half into a ball and flatten slightly. Wrap in cling film and refrigerate for at least 30 minutes.
Transfer one piece of dough to a lightly floured surface. Roll the dough from the centre to the edges to form a 30cm circle. Wrap the pastry around the rolling pin. Unroll it onto a 23cm pie dish. Ease the pastry into the pie dish, being careful not to stretch it. Trim the pastry evenly with the rim of the pie dish; return the pastry-lined pie dish to the fridge.
In a large mixing bowl, stir together the sugar and cornflour. Add the strawberries, raspberries and blueberries; gently toss until berries are coated. Allow fruit mixture to stand for about 15 minutes.
Preheat the oven to 190 C / Gas 5. Place a baking tray in the oven to preheat.
Roll out the remaining pastry for the top. Stir the berry mixture and pour the filling into the pastry-lined pie dish. Place the top layer of pastry over the pie and trim the edges, leaving a 1cm overhang. Fold the top pastry under the bottom layer of pastry, pressing lightly to seal. Crimp the edges of the pie and cut vents in the top to allow steam to escape. To prevent over-browning, cover the edge of the pie with foil.
Bake in the preheated oven on the baking tray for 25 minutes. Remove the foil.
Bake for an additional 20 to 30 minutes, or until the filling is bubbling and the crust is golden. Cool on a wire rack.
You can use frozen fruit instead of fresh berries; just increase the bake time. Bake the pie for 50 minutes, remove the foil; bake for an additional 40 minutes or until filling is bubbling and pastry is golden brown.