Place quartered apples in a bowl, and gently mix them.
Combine cinnamon and sugar in another bowl.
Sprinkle one-third of the sugar mixture over half the apples.
Add blackberries, and sprinkle with the next one-third of the sugar mixture.
Cover with remaining apples, and add remaining sugar-cinnamon mix.
For the crumble topping: Mix flour, cinnamon, sugar, and oats.
Cut butter into small cubes, and rub into mixture until breadcrumb texture.
On top of the fruit, spread the crumble. Turn the oven on to 170 degrees Celsius (150 degrees fan, gas 3).
Until golden and bubbling, bake crumble for 40 minutes.
Watch for golden sugar, not black. Cover with foil if needed.
Serve with homemade custard, if desired.