Mix the flours and salt in a bowl, then grate in the butter. Distribute the butter gently through the flour, using a round-bladed knife, then add the measured water and mix to a fairly firm dough. Put the pastry into a polythene bag and chill in the refrigerator for 1 hour, if possible.
Roll out the pastry thinly on a lightly floured surface. Cut out 12 rounds using a 8 cm (3½ inch) fluted cutter and 12 rounds using a 5 cm (2 inch) fluted cutter. Line a 12-section bun tray with the larger rounds.
Mix the apples and marzipan together in a bowl. Spoon the mixture into the pastry cases, packing it in well.
Brush both sides of the remaining rounds with milk and lay them on top of the tartlets in the tin. Press the edges together to seal and sprinkle each one with a little demerara sugar, if liked. Bake near the top of a preheated oven, 220°C (425°F), Gas Mark 7, for 15–20 minutes until golden brown. Remove carefully from the tins and leave to cool slightly on a wire rack. Serve warm or cold.