Preheat oven to 190 C / Gas 5. Grease a 20x30cm tin.
Combine 200g of the sugar, flour, baking powder, salt, ground cinnamon, margarine and egg. Dough will be crumbly. Pat just half of the dough into the prepared tin.
Combine the remaining 100g sugar, cornflour and berries. Place mixture over the base in the tin. Crumble the remaining dough over the tops of the berries.
Bake at 190 C / Gas 5 for 45 minutes or until the top is slightly brown.