1. Preheat the oven to 170oC/330oF/gas 3.
2. Mix the cinnamon and Demerara sugar in a bowl. Core and quarter the apples, then put half in an oven dish and sprinkle with half the sugar mixture. Add the blackberries and cover with the remaining apples. Sprinkle with the remaining sugar and cinnamon mixture.
3. Next, make the crumble topping. Put the flour and cinnamon in a bowl and mix well, then stir in the sugar and Quaker Oats. Cut the butter into small cubes, add to the mixture and rub it in with your fingertips until the mixture is the texture of breadcrumbs. Lay the crumble mixture on top of the fruit.
4. Place the dish in the oven and bake for about 40 minutes . It is ready when the crumble is golden, serve with custard.