Why did carol drinkwater leave all creatures great and small?

According to the Wikipedia article on the : "In the revived series, Lynda_Bellinghamtook over the role of Helen (following Carol Drinkwater's decision not to continue in the part, as a result of the ending of a real-life romance with Christopher Timothy).

http://en.wikipedia.org/wiki/All_Creatures_Great_and_Small_(TV_series)