We're at the Anniversary Games Diamond League meeting in London, to meet athletes preparing for next month's World Athletics Championships in Beijing. Usain Bolt talks about the challenge of racing against athletes who have previously been banned as drugs cheats. We also hear from two of the USA's Olympic champions Sanya Richards-Ross (400m) and Aries Merritt (110 m hurdles) and two of this year's form athletes likely to be challenging for medals in Beijing, Wayde van Neikerk (400m, South Africa) and Mercy Cherono (5,000m, Kenya).