Blackberry and Apple Crumble

Not long left for blackberry picking! Crumble is not always my forte even though it must be one of the simplest things. Thanks to an article in the paper, I have mastered the art of a good crumble topping.
Ingredients
Fruit
650g apples, chopped
3 handfuls of blackberries
60g brown sugar
1 1/2 tsp cinnamon
Topping
200g unsalted butter
250g demerara sugar
100g plain flour
100g oats
100g …