Place the flour into a large mixing bowl. Cut the cold butter into small cubes and add them to the flour. Rub the butter gently into the flour, using your thumbs and forefingers, taking care not to overwork. Stop when you still have some small lumps of butter remaining - roughly the size of peas.
Add the milk (or water) and stir roughly with a round-bladed knife. Using your hands, gently push the mixture together to form a rough dough and then turn out onto a very lightly floured work surface. Gently work into a smooth dough by pressing with your fingers and folding the dough onto itself a few times. Don't overdo this. You just want to make sure that the dough is uniform in appearance. Flatten into a disc and wrap in greaseproof paper. Place in the fridge to rest for about an hour.
After the dough has rested, roll out so that it's just larger than your baking tin (I used a 12x8" rectangular flan tin but you could easily use a round 9" or 10" tin). Roll the pastry back up over your rolling pin and use this to transfer it to the tin. Starting at one end, unroll the pastry so that it is draped over the tin. Gently press the pastry into the tin. Try not to stretch the pastry - if you lift the edges you can press it into the tin without stretching. Leave the excess pastry overhanging the edge of the tin. Place back into the fridge to rest for at least 20 minutes, but up to an hour is preferable.
Preheat the oven to 180ºC.
Once the pastry case has rested, place a piece of greaseproof paper inside. Fill this with baking beans, or rice, and pop into the preheated oven to blind bake for 10-15 minutes, or until the base is beginning to set and the pastry is holding its shape. After this time, remove the beans and paper and return to the oven for 5-10 minutes, or until the pastry has dried out and is beginning to turn golden. Set aside in the tin and leave the oven on.