Preheat the oven to 180°C, fan 160°C, gas 4. Put the flour, butter and sugar into a food processor and whizz to crumbs. If you don't have a processor, put the flour into a large mixing bowl, add the butter and rub it in lightly, using your fingertips. When it's crumbly, add the sugar and combine.
Arrange the blackberries and apples in a round, shallow, ovenproof baking dish or in 2 large and 2 small ramekins. Drizzle with the orange juice and sprinkle over the caster sugar. Cover with the crumble mixture. Place on the middle shelf in the oven and bake for 40 minutes, until the crumble is golden on top and the fruit is tender (ramekins will take less time, so keep an eye on them).
Cook's tip: this classic pudding is delicious on its own, but why not try serving with creamy custard?