Zara Tindall has broken her silence on her nerve-racking sport following Princess Anne’s head injury. The 43-year-old has given an insight into the world of horse racing in a new documentary. This comes as her mother, the Princess Royal, was hospitalised last month after suffering a blow to the head from a horse on her Gatcombe Park estate. Anne, 73, spent five nights in hospital and did not return to royal duties until July 12. Her daughter, Zara, is now appearing in a new documentary discussing why equestrians compete at the Cheltenham Festival, one of the most popular horse-riding events in…