Place the flour, butter and caster sugar in a large bowl or food processor, Rub the butter into the dry ingredients or whizz together until it resembles breadcrumbs. Pat together to form a ball.
On a lightly floured surface, roll the biscuit dough out to a circle measuring 15 cm. Carefully lift the disc onto a non- stick baking sheet.
Crimp the rim with your fingers to create a fluted edge, mark into 8 triangles and then prick all over with a fork. Sprinkle with demerara sugar, then chill for 20 minutes. Preheat the oven to 180°C, fan 160°C, gas 4.
Bake for 20-25 minutes, until just turning golden. Remove from the oven and leave to stand for 5 minutes, carefully cut to create 8 wedges. Cool the biscuits on a wire rack.