The week's best chefs return for the quarter-finals and this is where the battle really begins. Their first challenge is an Invention Test - but this one has a twist. The quarter-finalists must create an exceptional plate of food from the kitchen offcuts and trimmings. Gone are the array of quality ingredients they are used to cooking with. Instead, the judges ask them to make their dishes from kitchen scraps, including bones, trimmings, fish skins, over-ripe fruits, cheese and stale bread.