Hi,
From the census it would appear that Elizabeth Foster (nee Benson) was born c.1858, Easton on the Hill, Northamptonshire.
Struggling to find an Elizabeth Benson on earlier census however there is an Elizabeth Henson that could fit -
1861 - 1871 she is living with her parents William and Eliza.
RG 9 Piece 2313 Folio 39 Page 6
RG10 Piece 3309 Folio 41 Page 2
In 1881 Elizabeth is a servant living in Uppingham, Rutland
RG11 Piece 3190 Folio 15 Page 24
I cannot see a marriage yet for an Elizabeth Henson/Benson to James Foster.
The birth of Beatrice and her brother George William has their mother's maiden name as Benson, however the names Benson and Henson are very similar. Therefore perhaps worth looking further.
JJ