Identified in the 1851 census for Warwickshire as living at Coleshill, Nr. Birmingham, Warwickshire, Servant.
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I have it from Michael Gadd, an Underhill Family researcher that William Knibbs & Eliza Lock appear on the 1871 Census as
Knibs in Sheldon, he is then 57 birthplace not known and she is 45 born at Balsall St. (Balsall Common is not far from Hampton in Arden). The household looks a bit complicated:
Fanny Viner (or Vines) 11 Sheldon
William Knibs 9 Sheldon
Harriet Knibs 6 Sheldon
Clarah Knibs 1 Sheldon
George Knibs Gson 1 Meriden
Roseanah Knibs Gdau 2mo Meriden
(spelling is as it appears on the image - not a transcription).
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The 1881 British Census shows William living at the Cottage, Sheldon, Warwickshire, England with his second wife Eliza, Emma and George (two of Eliza's children from her first marriage) Albert, Florry and George VINNS.
Albert is the youngest child of William and Eliza. Florry, I believe from information passed to me by Hilda Knibbs, was an illegitimate daughter of one of the Viner girls, but was brought up by Eliza. I can only assume that George VINNS (surname as given in the census transcription) was another illegitimate child of one of the Viner girls.
William KNIBBS Head M Male 63 Balsall, Warwick, England Ag Lab
Eliza KNIBBS Wife M Female 63 Balsall, Warwick, England Wife
Emma KNIBBS Daur U Female 29 Sheldon, Warwick, England Visitor
George KNIBBS Son U Male 25 Sheldon, Warwick, England Ag Lab
Albert KNIBBS Son Male 8 Sheldon, Warwick, England Scholar
Florry KNIBBS Daur Female 4 Sheldon, Warwick, England Scholar
George VINNS Grandson Male 11 Sheldon, Warwick, England Scholar

Emma and George are listed as Knibbs so we can assume that they were legally adopted by William as his own.
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There remains some confusion over the birth place of William and Eliza. William we believe is from Woodstock in Oxfordshire and Eliza from Hampton in Arden, Warwickshire. The 1891 census confuses things.

I have the following from Michael Gadd, an Underhill Family researcher:

Quite probably, Sarah is the daughter of Joseph Underhill and his wife Sarah Smith who appear in the 1841census for Fillongley, Warwickshire, England.
Surname First name(s) Sex Age Occupation Where Born Remarks
UNDERHILL Joseph M 70 Farmer Warwickshire
SMITH Sarah F 40 Housekeeper Warwickshire
UNDERHILL Mary Ann F 12 Warwickshire
UNDERHILL Eleanor F 10 Warwickshire
UNDERHILL Maria F 6 Warwickshire
UNDERHILL Joseph M 5 Warwickshire
BURTONWOOD Sarah F 15 Female Servant Warwickshire

Joseph was buried at Fillongley on 16th June 1849 aged 83 and Sarah appears on the 1851 Census at Big House, Fillongley, as Sarah Underhill, age 52, with her younger children Ellen, 21, Maria, 16, and Joseph, 14. So, whilst at the 1841 census return, Sarah appeared as his housekeeper, I suspect she was more than that.

The IGI christenings show that Joseph & Sarah had at least 6 children
together:
John Underhill Smith 26th Oct 1821 - buried 23rd May 1840 age 18
Sarah Smith 20th Dec 1821
Mary Anne Underhill Smith 15th Mar 1829
Elianor Smith 14th Jan 1831
Maria Smith 31st Jan 1835
Joseph Smith 16th April 1836

From the transcript they appear to be recorded as the children of Joseph (no
surname) and Sarah Smith.