Thanks for these suggestions.
I've used the address search for 1891 and found the family name has been mistranscribed, so I've submitted corrections.
I can't use address search for 1901, as it's the name of a house where she was in service. I searched under the details of the head of the household, Arthur Benson, born 1853, living in Buckinghamshire, and added the name "Ada" (one of his daughters) as another person in the house. It brings up the correct record, and lists my ancestor as Ellen L Grove, born 1875 in Wandsworth, employed as a parlourmaid.
But then when I search using the details "Ellen L Grove, year of birth 1875, place of residence Buckinghamshire" it finds no records! And I did try clearing my cookies between searches as I've found FindMyPast is being a bit glitchy. Do they not make details of servants searchable or something?
Lady N