Heat the oven to 425°F/220°C and grease and flour a baking sheet. Sift the flour into a bowl with the sugar, baking soda and salt. With your fingers or two knives, work in the shortening until the mixture resembles coarse crumbs. Stir in the currants and make a well in the center of the mixture. Stir the cream of tartar into the buttermilk or milk and add to the well. Stir lightly, just until the dough clings together. It should be soft and sticky, so add more liquid if necessary.
On a floured board knead the dough lightly for 1 minute. Pat it to 1in/2.5cm thickness. Stamp out rounds with a pastry cutter (above). Transfer the rounds to the baking sheet and bake in the preheated oven for about 15 minutes, until lightly browned and a skewer inserted in the center comes out clean.