These popular Christmas specials serve as extensions to the beloved British      series that's based on the real-life adventures of veterinarian James Herriot.   In the first, James returns to Darrowby following his wartime service in the     RAF, and finds it difficult to adjust. In the second, Siegfried and James      dwell on whether the countryside can resist the oncoming tide of progress.     Great Britain, 1983-85, 180 mins.