It's week four of The Great British Bake Off, just nine bakers remains, and - for the very first time - it is Batter Week. Mary and Paul have set three challenges to test the bakers on some store cupboard classics. To start things off there is a British favourite. It may sound simple, but the judges are looking for perfection - a uniform bake across the batch and a tasty savoury filling. The bakers really have to rise to the occasion in this signature challenge.