For the pastry, 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. Be careful to keep the butter in lumps.
Mix 180-200ml/6-7fl oz chilled water and lemon juice together in a jug and gradually 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, or until the dough comes together. The dough will be very wet at this point.
Tip the dough onto a well-floured work surface and quickly shape it into a rectangle about 30x20cm/12x8in.
With the pastry vertically on the board in front of you, fold the bottom third of the pastry up onto the middle third, then the top third down onto the other thirds. Rotate the pastry a quarter turn and repeat the folding.
Wrap in cling film and leave to chill in the fridge for 10 minutes.
Take the dough out of the fridge and, with a rolling pin, roll into the original narrow rectangle and repeat the folding, the turn and the rest as before. Do this three times in total. Don't worry if the butter oozes through the dough at first, just keep the board and rolling pin well floured.
For the filling, soak the currants in the sherry in a bowl.
Melt the butter in a saucepan and stir in all the other filling ingredients, including the currants and sherry. Leave to cool.
To assemble, preheat the oven to 220C/425F/Gas 7.
Roll the pastry to the thickness of 3mm. Cut out 6 x 12.5cm/4¾in rounds. Cover the rounds in cling film and leave to chill in the fridge for 10 minutes.
Place a good teaspoon of filling in the centre of each pastry round. Pull the pastry around the filling and squeeze to seal. Trim away any excess pastry. Turn the balls over and place on a baking tray. Flatten lightly with a rolling pin until the fruit begins to show through the pastry.
To assemble, in a bowl, beat the egg white lightly with a fork, until frothy. Brush the tops of the cakes with this and sprinkle with sugar. With a sharp knife, make three small parallel cuts on the top.
Bake for 20 minutes, or until the pastry is lightly browned and puffed up.