Sift the flour and salt into a large bowl and add the butter. Mix the butter around with a large metal spoon to coat it in flour.
Mix the water and lemon juice together in a jug and pour it into the flour and butter mixture.
Using a round-tipped knife, cut across the contents of the bowl several times, turning the bowl continuously as you chop the butter into the flour, until the dough comes together.
When the dough has come together as a loose lump, tip it onto a lightly floured work surface and quickly shape it into a thick slab, roughly the dimensions of a house brick.
Flour the work surface well and roll the dough out to a rectangle approximately 38cm/15in x 20cm/8in.
Fold one-third of the dough to the centre, then fold the other third over that. Roll the pastry out again to the same 38cm/15in x 20cm/8in proportions. Fold in exactly the same way again.
Rotate the pastry a quarter turn and repeat step 6. Continue rolling, folding, pressing and turning a quarter-turn 3-4 more times. Don't worry too much if the butter oozes through the dough occasionally in the early stages, just keep the board and rolling pin well floured.
After the last folding stage, wrap the pastry in cling film and place into the fridge to chill for several hours, preferably overnight.
When ready to use, if the pastry is very firm, allow it to warm in the room for a few minutes before using. (NB: wrapped puff pastry will store well in the freezer for up to six weeks.)