In the mountains of Mexico, there is a ritual dance that is so extraordinary that the United Nation's cultural organisation has given it protected status. It takes place at the top of a 30 metre pole, with five dancers in brightly coloured costumes with tassels on. They then leap off, attached only by a long thin cord. It is spectacular. It used to be part of a fertility ceremony, and the people who perform it are called the Voladores of Puebla, or "flying people". When they performed in London, our reporter Judi Herman went to meet two of the . . .