Put the flour and salt into a large mixing bowl and stir
Chop the butter into 1cm cubes and add to the bowl. Toss with the flour to coat each cube
Mix the lemon juice with the water and pour 3/4 into the bowl. Mix with a blunt knife, incorporating all of the flour as you go, until the dough comes together in a soft ball. Add more water if necessary, but take care not to add too much. The dough should neither be dry, wet or sticky
Dust the worktop & your rolling pin with flour, then roll out the pastry dough to form an oblong about 2cm thick. Lift one short edge of the pastry and fold it over the middle, then do the same with the other edge of the pastry to make three layers.
Turn the pastry 90 degrees (a quarter turn), then re-roll and repeat the folding process. Repeat once more, then wrap in cling film and chill for 30 minutes
Unwrap the pastry and roll out again, fold and chill for a further 20 minutes
Repeat this process once more, wrap and chill for a final 20 minutes