Preheat oven to 350 degrees. Prepare 9″ x 13″ baking pan.
Bring currants and water to a boil. Mix together cornstarch and 2 tbsp. water and slowly stir into boiling currants. Continue stirring until thickened, about 3 minutes. Remove from heat; stir in zest and vanilla.
Combine flour, baking soda, porridge oats and sugar. Add butter and mix thoroughly.
Press 2/3 of the crumb mixture into the pan.
Spread currant mixture over bottom layer. Sprinkle remaining crumb mixture evenly over the currant layer.
Bake for 30-35 minutes or until lightly browned.