Preheat the oven to 200°C/180°C fan and line a 20" square baking tin.
Place your berries into a colander and run under a warm tap for a minute and leave to drain for around 30minutes until softened. Squeeze out as much excess water as you can.
Mix the berries in a bowl with the caster sugar, cornflour and plain flour.
In a separate bowl take the oats, flour, brown sugar, baking powder, melted butter and mix until it comes together in a crumble-like mixture.
Firmly press around two thirds of the crumble in to the prepared tin, making sure to cover the entire base. Bake fo 10 minutes.
Remove from the oven and carefully arrange the berry layer on top of the bottom layer.
Sprinkle the remaining crumble mix over the top and bake for a further 10 minutes until turning golden.
Remove from the oven and leave to cool. Once cooled, chill in the fridge for around two hours and then cut into bars.
Put the kettle on and enjoy :)