Preheat the oven to 200°C, fan 180°C, gas 6. Roll out the pastry until it’s about 3mm thick. Using a round cutter, cut out 10 bases and place in a muffin tin. Re-roll the remaining pastry and cut out 10 smaller discs for lids and keep to one side.
Mix together the apple and marmalade and divide between the pastry cases, lightly brush the smaller disc edges with water and press firmly onto each mini pie.
Make a small hole in the top of each pie to allow the steam to escape, then brush with water and sprinkle with the caster sugar. Bake for 18-20 minutes until pastry cooked and golden. Remove from the oven and scatter with grated orange zest. Serve warm with custard.