Preheat the oven to 180 degrees Celsius.

Put the blackberries, apple pieces and brown sugar into a saucepan. Put over a low heat and cook gently until soft. Remove from the heat and transfer to an ovenproof dish.

Put the butter and flour into a bowl and rub together between your fingertips until they resemble breadcrumbs. Tip in the caster sugar and stir with a wooden spoon until evenly distributed.

Sprinkle the topping evenly over the fruit filling. Put in the oven and cook for 30 minutes, or until bubbling and golden brown.