Mrs. Hall
Many comments have been made regarding how the loss of Carol Drinkwater and the infrequent appearances of Peter Davison impacted on the later series. Certainly some of the "magic" was missing. However I believe that insufficient recognition has been given to Mrs. Hall's character in the first three series.
Unfortunately Mary Hignett died soon after completing the third series. There was a fleeting tribute from the cast in the 1983 special when they focussed on a photograph of her. We will all miss her.