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 C / Gas 4. Lightly grease a 9x17.5cm baking tin.
Combine 375g flour, 200g sugar, baking powder and bicarbonate of soda in a large bowl. Rub in butter until mixture resembles coarse sand. Beat together eggs, buttermilk and vanilla in a separate bowl. Stir egg mixture into flour until just moistened. Spoon two-thirds of the mixture into prepared cake tin; evenly spread on the cooled filling, then cover with remaining mixture.
For the topping: combine sugar and flour; rub in butter until the mixture resembles coarse crumbs; sprinkle over the top of cake. Bake in preheated oven for 45 to 50 minutes or until golden brown. Cool before serving.