Take butter out of the fridge prior to making, so that it becomes room temperature.
Add all ingredients to a bowl. Rub butter through the dry ingredients until it resembles very fine breadcrumbs (it will take a while, but is well worth it). Then press together to form a very firm dough. If it's not coming together easily, add water one teaspoon at a time. You do not want the dough to become claggy or sticky. Roll into a ball & cover in cling wrap & refrigerate 40 minutes or overnight.
Line a pizza tray with baking paper.
Place dough in centre of tray and, with a rolling pin, gently roll it out into a circle, taking up most of the tray (leaving about 2 - 3 cm gap from edge of tray).
Prick all over with a fork.
Gently press fork around edges to give a pattern to the shortbread.
With a knife, cut the flattened circle in half, then in half again, and in half again & so on (like pizza slices...each wedge equates to one piece).
Bake 40 - 50 minutes at 160 degrees, until lightly coloured and crisp.
If preferred, currants can be pressed decoratively onto the slices, prior to baking.