Combine sugar and cornflour in a large saucepan; stir in rhubarb and strawberries and bring to a simmer over medium heat. Cook until thickened, about two minutes. Remove from heat, stir in lemon juice and let cool.
Preheat oven to 180 degrees C. Lightly grease a 20x30cm pan.
Combine 3 cups flour, 1 cup sugar, baking powder and bicarb soda in a large bowl. Cut in butter until mixture resembles coarse crumbs. Beat together eggs, buttermilk and vanilla in a separate bowl. Stir egg mixture into flour until just moistened. Spoon two-thirds of the batter into prepared pan; evenly spread on the cooled filling, then cover with remaining batter.
In a small bowl, combine 3/4 cup sugar and 1/2 cup flour and cut in butter until the mixture resembles coarse crumbs; sprinkle over the batter. Bake in preheated oven for 45 to 50 minutes or until golden brown. Cool on a wire rack.