Heat oven to 190C/170C fan/gas 5. Tip the plain flour and golden caster sugar into a large bowl. Add 60g unsalted butter, then rub into the flour using your fingertips to make a light breadcrumb texture. Then add the porridge oats and fold in.
Sprinkle the mixture evenly over a baking sheet and bake for 15 mins or until lightly coloured. Meanwhile, for the compote, peel, core and cut the bramley apples into 2cm dice. Put 30g unsalted butter and the golden caster sugar in a medium saucepan and melt together over a medium heat. Cook for 3 mins until the mixture turns to a light caramel. Stir in the apples and cook for 3 mins. Add the blackberries and ¼ tsp ground cinnamon, and cook for 3 mins more.
Cover, remove from the heat, then leave for 2-3 mins to continue cooking in the warmth of the pan. Add the fruit compote to an oven proof dish.
Sprinkle the crumble mixture on top of the warm fruit then put in the oven for 15 mins.