Apple and blackberry pie

Name: Apple and blackberry pie
Ingredients: 375g/13oz plain flour, plus extra for dusting; 200g/7oz salted butter, chilled and diced; 50g/1¾oz icing sugar; 1 large free-range egg, separated, plus 1 large free-range egg yolk; 2 Bramley apples, peeled, cored and diced; 1 lemon, zest and juice; 75g/2¾oz granulated sugar, plus extra for sweetening and dusting; 4 eating apples (a sharp, crisp variety such as Braeburn), peeled, cored and cut into large chunks or slices; 250–300g/9–10½oz blackberries (frozen is fine); vanilla ice cream, clotted or single cream, to serve