Pre-heat your oven to 180°c (or 160°c for a fan assisted oven or Gas Mark 4) and grease a 20cm by 28cm baking tray with a little butter. Set aside.
In a large bowl, using your fingers, rub the flour and butter together until it resembles breadcrumbs. Stir through the sugar before adding the egg. Mix until combined, adding a teaspoon of cold water as required and knead into a soft pastry dough.
Wrap the dough in cling film and place in the fridge to cool and firm up, whilst preparing the filling.
In a bowl mix the currants, sugar, butter and mixed spice until combined.
Remove the pastry from the fridge and roll to 0.5cm thickness and cut into two large rectangles, about the size of your tray. Place on pastry rectangle onto your pre-greased tray.
Spoon the currant mixture evenly onto the pastry on the baking tray, leaving a small gap around the edges. Wet the edges with a little water. Place the other sheet of pastry on top of the currants and gently press the edges together with your fingers, before finishing with a fork.
Brush the top pastry with a little milk or beaten egg, before pricking a few holes on the top of the pastry
Bake for 25-30 minutes until a light golden brown colour.
Remove from the oven and sprinkle a little sugar on top to finish. Once completely cool, cut into squares or slices of your desired size.