In a saucepan over medium heat, combine the cranberries, caster sugar and water. Cook, stirring occasionally until all of the cranberries have popped and the mixture is thick, about 15 minutes. Remove from heat and set aside to cool.
Preheat oven to 180 C / Gas 4.
In a large bowl, mix together the sponge cake mix, melted butter and eggs. Stir in the oats, dark brown soft sugar, ginger and cinnamon. Set aside about 185g of the mixture and spread the rest into the bottom of a 20x30cm baking tin. Pack down to form a solid base, getting it as even as possible. Spread the cooled cranberry mixture over the base. Pinch off pieces of the remaining mixture and place evenly over the cranberry layer.
Bake for 35 to 40 minutes in the preheated oven, until the top is lightly browned. Cool in the tin for about 40 minutes before slicing into bars.