Marking the 80th anniversary of Glyndebourne Festival, Glyndebourne: The Untold History, filmed this year, explores the beginnings of the opera house which sprang from the romance between founder John Christie and his opera singer wife, Audrey Mildmay, and provides an insider’s view on the company as it prepares to present a new production of Strauss’s Der Rosenkavalier, an opera which, like Glyndebourne itself, incorporates both tradition and innovation.