Roll out the pastry on a lightly floured surface and cut into 2 strips, 10 x 30 cm (4 x 12 inches). Space apart on a wetted baking sheet. Prick with a fork and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10–12 minutes until well risen.
Slice each strip in half horizontally, lift off the tops and place, baked side downwards, on a separate baking sheet. Bake all strips for 3–4 minutes more to dry out the soft centres. Leave to cool.
Whip the cream then fold in the custard and spoon over 3 of the pastry strips. Arrange the strawberries and raspberries on top of each strip and then assemble. Add the last pastry slice, if liked and dust with icing sugar. Transfer to a large serving plate. Cut each custard slice into 4 to serve. These are best eaten on the day they are made.