Pre-heat the oven to 180 C / 356 F / gas 4.
Line a baking tray with baking paper and place to one side.
Put the flour, salt and butter into a large bowl and, using your finger-tips, rub the flour and butter together until it resembles fine breadcrumbs.
Add the sugar followed by just enough milk to mix to a firm dough – it should not be sticky.
Wrap the dough in clingfilm and rest for 30 minutes in the fridge.
Tip the dough ball out on to a lightly floured surface and roll it out to a rectangle 5mm thick.
Sprinkle the chopped currants over half the surface and then fold the other half on top roll everything again so you end up with a rectangle roughly 18 by 28 cm trimming off any excess or uneven edges.
Cut this into fingers roughly 3cm by 7cm.
Place the biscuits on the baking sheet, prick with a fork in two rows, then brush with a little milk and sprinkle with granulated sugar.
Bake near the centre of the oven for 12–15 minutes, then place on a cooling rack to cool completely.