Place all filling ingredients in pan, and heat gently until mixture forms paste. Add more water if required. Leave mixture to cool.
Rub margerine into flour until it resembles breadcrumbs. Stir in oats and sugar, and mix well. place half the crumble mixture into the greased tray. Press down firmly. Smooth filling evenly over. leave a small gap to the edge of tin. spread remaining crumble evenly over the top.