Put the flour and caster sugar in a large bowl and whisk together. Add the butter and rub with the tips of your fingers into a sandy texture. Spread onto a baking tray and put in the oven for 30 minutes, stirring every 10 minutes.
Place the blackberries, granulated sugar, corn flour and water into a saucepan over a medium heat. Stir together and simmer gently for 10 minutes, until the fruit is beginning to break down and the liquid is thick.
Ladle the fruit mixture into serving bowls and sprinkle over the crumble before serving.