Sprinkle a clean work surface evenly with the granulated sugar and lay the pastry sheet over the sugar, pressing down to coat. Spread the jam in a thin, even layer over the unsugared side of the pastry, then, starting with a long edge, roll up the pastry.
Cut the pastry into 24 slices, each about 1 cm (1/2 inch) thick, and arrange on 2 baking trays lined with baking paper. Bake in a preheated oven, 200˚C (400˚F), Gas Mark 6, for 12–15 minutes or until crisp and golden. Carefully transfer to wire racks to cool slightly before serving.