By Nicola Methven Jayne Torvill and Christopher Dean’s decision to hang up their skates could be the beginning of the end for Dancing on Ice as ratings keep tumbling. The pair marked the 40th anniversary of their stunning win at the Winter Olympics in Sarajevo by announcing their retirement from performing together. It came as the ITV series got its smallest ever audience, with just 2.9 million viewers watching the celebrity skaters on January 28. It is far short of the 12 million who tuned in during the show’s heyday and will come as a big blow to long-term presenter Holly Willoughby, who ret…