Put the blackberries into water and leave for an hour to wash them and ensure that no bugs have survived. Strain, place them in an ovenproof container and cover with half the sugar. Sift the flour into a bowl and rub in butter, then slowly add the remaining sugar. Pile the mix over the fruit and pop in an oven at 200oC/400oF/Gas 6). Bake for 35 mins.