The four best chefs from this week's heats return for their quarter-final, where they face two more demanding challenges set by Monica Galetti and Marcus Wareing.First, the chefs face an invention test inspired by the seaside. They are presented with a choice of fish, shellfish and a selection of vegetables to use. They have 10 minutes to select the ingredients and 70 minutes to cook. There's a lot to prove with this dish, as the judges want to see that the chefs can produce refined food with an imaginative twist.Then the chefs cook for two of the UK's toughest restaurant critics, Grace Dent and Tom Parker Bowles. Will the critics' taste buds be satisfied, or will the food leave them with a sour taste? Only the three most talented chefs will go through to the semi-finals and continue their pursuit of the MasterChef: The Professionals title.