Dai Davies, the former head of royal protection for the Metropolitan Police, says accidents like these are embarrassing for the service, but that training for officers is very rigorous.
More top news
Former rugby star Ian Gough has won an assault appeal against former Miss Wales and ex-girlfriend Sophia Cahill.
Party leader Nick Clegg takes questions from Cardiff factory workers as he launches Welsh Liberal Democrats' election campaign
Mark Jones is accused of the murder of five-week-old Amelia who he was babysitting while her mother went to watch a film.