Put the chopped apples in a saucepan, add the blackberries, lemon juice and enough water to half fill the pan. Simmer for about 20 to 30 minutes, apples should be softened but not pureed, blackberries will have just about vanished but coloured the mixture.
Preheat the oven to 150 C / Gas 2. Grease an 18cm baking dish.
While the fruit is simmering mix the flour, granulated sugar, demerara sugar, cinnamon and margarine together until it resembles coarse breadcrumbs.
Once the fruit mixture has softened enough spoon the mixture into the prepared baking dish. Add the crumble mix over the top, level but do not firm the crumble mix.
Bake in the preheated oven for half an hour until the fruit is piping hot and the crumble is golden brown. Alternatively you could freeze the crumble until required, heat for 40 minutes if using from frozen.