Xi Jinping has become the most powerful Chinese leader for decades. He is a man the British government wants as a major partner. But on the eve of President Xi's state visit, Panorama reveals what sort of friend he really is. The BBC's China editor Carrie Gracie retraces Xi's extraordinary journey from cave dweller to absolute power. It's a story that the Chinese government doesn't want told - Xi emerges as a ruthless political operator, who has crushed opposition at home and seeks to punch China's weight abroad.