Put the flours and salt in a large bowl, add the butter and rub into the flour between your thumbs and fingertips until the mixture resembles coarse breadcrumbs. Mix the lemon juice and water, pour over the flour and butter mixture and stir in quickly with a round-ended knife. When the dough starts to cling together, discard the knife and use the fingers of both hands to press the mixture gently together into a ball. Knead the dough lightly by pressing and turning it in the bowl until fairly smooth, then place it on a sheet of plastic wrap, press it into a disc and wrap tightly to exclude the air. Rest the dough for 20 minutes in the refrigerator before rolling out. After rolling out and lining the pie plate or tin you are using, chill for 1 hour before baking.
For sufficient dough to line a 25cm pie plate or tin, or for a two-crust 20–23cm pie, use 1 1/4 cups each of plain and self-raising flour, 130g of butter and 4 tablespoons of iced water (the pinch of salt and the teaspoon of lemon juice remain the same).