pbr Prince William was joined by fiancee Kate Middleton at a Christmas charity variety show.br ppbr It was the couple's first joint engagement since plans for their wedding were announced last month.br ppbr Miss Middleton walked by the prince's side as the couple arrived for the concert, staged to raise funds for the Teenage Cancer Trust.br ppbr William joked with onlookers, telling one group of teenagers dressed as elves: "Cool hats".br ppbr Miss Middleton waved and smiled at well-wishers but said nothing.br ppbr William was guest of honour at the fund-raising gala in Thursford in Norfolk, which showcased a cast of professional singers, dancers and musicians.