The oldest folk festival in Britain, the Sidmouth International Festival, celebrates its 50th anniversary this year. Its theme is "generations", marking the fact that so many famous artists have children who carry on the tradition. Mike Harding hears from a number of folkie families including Martin Carthy and Norma Waterson with their daughter, Eliza; Kate Rusby, with her parents Steve and Anne; Jim Moray with his parents Colin and Jane; and John Kirkpatrick with his son Ben.