Why was cicely courtneidge replaced in on the buses?

Cicely Courtneidge On The Buses Because Doris Hare was always 1st choice, but was not available for series 1. Cicely was used for the part in series 1 with the understanding that if On The Buses had a second series, she would be replaced by Doris Hare.

From Cicely Courtneidge fan page on Facebook.