Sift the flour, cream of tartar, bicarbonate of soda and salt into a mixing bowl and rub in the butter with your fingertips until the mixture resembles breadcrumbs. Stir in the sugar and add enough milk to mix to a soft dough.
Turn on to a floured surface, knead lightly and roll out to 2 cm (¾ inch) thick. Cut into 5 cm (2 inch) rounds. Place on a floured baking sheet and brush with milk.
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes. Transfer to a wire rack to cool. Serve with butter and jam or with whipped cream and jam.