Lay the pastry sheet on a baking sheet lined with baking paper and trim each corner to make a roughly round piece of pastry.
Mix together the apples, raisins, brown sugar, melted butter and cinnamon in a bowl until evenly combined. Spoon the apple mixture on to the pastry sheet, arranging it in a circle, leaving a 2.5 cm (1 inch) border. Pull the pastry edges up and over the filling to make a rim. Brush the pastry with the milk and dust with the icing sugar.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes until the pastry is golden and the fruit softened. Dust with extra icing sugar and serve warm with custard (see below).