Julian Fellowes personifies the success of British film on the large and small screen. Yet he toiled as a mid-level actor and thwarted writer until Gosford Park won an Oscar for Best Original Screenplay. Further screen achievements include Vanity Fair and Young Victoria and a TV drama about the Titanic. However, it is as creator and writer of Downton Abbey, winner of this year's National Television Best Drama Series Award, that he has acquired his greatest fame. Juliet Nicolson, social historian and author of Abdication, talks to him about being a late bloomer and a chronicler of the class system.