Mary King is a sportswoman like few others. She's 50-years-old and still at the pinnacle of her sport. That sport is three-day eventing. Next year, she's hoping to collect her first Olympic gold medal - at what will be her sixth games. But is equestrianism - as its enthusiasts insist - the toughest sport in the world?Or is it less a test of the person - more of the horse - and a preserve of the rich?(Image: Mary King. Credit: Press Association)