Preheat the oven to 200°C about 15 minutes before baking. (It will take that long to make the pie)
1. Peel and core the apples, cut into pieces and then put them in a bowl of water with a little lemon juice to stop them turning brown.
2. In a large mixing bowl, mix together the brown sugar, cinnamon and flour.
3. Place a third of the apple pieces in the base of a deep 23cm pie dish so that it is completely covered. Sprinkle half the sugar mixture over the apples in the pie base. Cover with half the remaining apples and sugar then arrange the rest of the apples on top.
4. Cut the orange in half and squeeze out the juice using a lemon squeezer. Pour the orange juice over the apples. Cut the butter into small pieces and spread the bits evenly over the apples.
5. Roll the pastry out on a lightly floured surface until it is just larger than the pie dish. Avoid making it very thin as you won't be able to move it to the pie... Dampen the pastry edge then stick it on to the dish, pressing the edges firmly down. You can decorate the top of the pie with any spare pastry. Brush the pastry with a little water and sprinkle with the caster sugar. Bake in the oven for 15 minutes then reduce oven temperature to 180°C and continue to cook for 30 minutes or until the pastry is golden brown.