Hi Jacqui,
Was she born around 1845?
If so, I can see a possible in 1861 (she's a servant).
This would be her in
1851HO107/1891 141 8
Knowstone, Devon Edward Marshall, head, 60, Farmers lab
Betty, wife, 57
Mary Ann,
dau niece, 10
Solomon, do. (=niece?), 7
Elizabeth, do., 6Rhoda, do., 4
John, do., 7
Everybody born Knowstone.
Tanja