Place the flour and salt in a big bowl. Using a box grater, grate the butter into the flour. Add the water and gently mix to a dough. Don't rub the butter into the flour we want to keep them whole as they will help the pastry to puff. Wrap in cling film and place in the fridge for 30mins.
Remove and on a floured surface roll out to a large thin rectangle. It will be really raggedy and have patches of butter. Fold up like a letter into three, turn 90 degrees and roll again the other way to a large rectangle, fold up again and fold the opposite way to the first fold. Wrap and chill for 30mins.
Remove and roll again and fold, turn and roll again and fold. Wrap and place in the fridge. Do this one more time so the pastry has had 6 folds. It should now be smooth and glossy and the butter will be in lots of layers so that when it bakes it should rise a small way and be really buttery and flakey.