S15, E3: The cars put to the test include the Aston Martin Rapide, the Porsche Panamera Turbo and the Maserati Quattroporte, in a search for the world's greatest four-seat supercar. Richard is also on the test track the Chevrolet Camaro SS and Mercedes E63 AMG. There's a double dose of celebrity as `Harry Potter' star Rupert Grint and F1 ace Rubens Barrichello take their turns as the stars in the Reasonably Priced Car.