Preheat oven to 160C and line a square 8inch x 8inch baking tray with baking paper.

In a large bowl combine the plain flour, oats, baking powder and sugar. Then mix in the butter to form a firm, but slightly crumbly dough. Press half of the mixture into the baking tray and then bake in the over for 7-8mins. Remove from the oven and allow to cool slightly.

Meanwhile, prepare the filling. Add the frozen berries to a saucepan and heat on a low heat for 5-10mins until they begin to break down. Stir in the arrowroot powder and caster sugar and then bring to the boil. Reduce and allow to simmer for 3min.

Spread the berry mixture over the base and then crumble the remaining dough over the top. Return to the oven for 15-20min until golden brown. Remove from oven and allow to cool completely before cutting into squares.