The outfits for the nannies are supposed to be an homage the signature outfit of Mary Poppins (1964).
Yvonne Shove was the original third nanny for the show before she was replaced by Yvonne Finnerty (guest star Jo Frost would sub in for her before the arrival of Yvonne Finnerty). She was replaced because of pregnancy.
Due to budget constraints, Nanny Yvonne is not present in the 4th season.