On 16 August 1819, troops charged the crowds in St Peter's Field - 18 people lost their lives and around 700 were injured. Within days, the press were referring to it as "The Peterloo Massacre" after the battle of Waterloo just four years earlier. The events shocked the nation and eventually led to widespread change. Katharine Viner meets descendants of those there that day, she looks at the background and build up, hears graphic accounts of the slaughter, death and injury and examines how the events would revolutionise what was meant by democracy.