Sift the flours into a bowl and add the sugar. Work in the margarine with your fingertips – knead well.
Pack into a floured shortbread mould then turn out onto a baking sheet. Alternatively, pack into an 7 inch ( 18 cm) sandwich tin.
Prick all over with fork, and pinch up the edges decoratively with finger and thumb.
Bake for 40 minutes until firm and pale golden.
Mark the 8 triangles while still hot.
Cool slightly before transferring onto a wire rack.
When cool, dredge with sugar. Serve cut into wedges.