Heat the oven to 190°C/170°C fan/gas 5. For the crumble topping, sift the flour and a pinch of salt into a mixing bowl, add the butter and rub the mixture together with your fingertips until it resembles breadcrumbs, as for pastry. Continue rubbing in until the mixture begins to stick together to form small pea-size clumps. Stir in the demerara sugar.
Peel, quarter and core the bramley apples, then thinly slice them into a large mixing bowl. Peel, quarter and core the eating apples, then cut them into slightly thicker wedges. Add to the bramleys with the blackberries and mix together.
Mix the cornflour with the caster sugar, add to the bowl of fruit and toss together well. Set aside for a few minutes until the fruit juices begin to seep out and make the sugar damp, then toss everything together once more until the cornflour mixture clings to the outside of the fruit.
Spoon the fruity mixture into the baking dish, then pack it down into an even layer with your hands or the back of a spoon.
Spoon the crumble mixture in an even layer over the top of the fruit. Bake for 45 minutes until the topping is richly golden brown and the fruit is bubbling hot and tender.
Remove the crumble from the oven and leave to rest for 5-10 minutes. Serve hot with custard, crème fraîche, cream, ice cream or yogurt.