William married using the surname of Harris.

William is the illegitimate son of William Harris of Sibford and Sarah KNIBBS of Swerford.

There's a mystery surrounding William's surname. We know he was baptised at Swerford, Oxfordshire on 22 Jan 1822 as:
KNIBBS/HARRIS William bastard s. William HARRIS, Sibford & Sarah Knibbs, Swerford, farm servants

We can see from the birth certificate of his older daughter Caroline that he was using the name William Harris when she was born in 1846. Her birth was registered as Caroline Harris, the daughter of William Harris and his wife Emily Harris formerly Davis.

However, the family was quite clearly using the surname Knibbs at the time of the 1851 census.
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1851

The 1851 census shows William and his wife Emily, and daughter Caroline living at Swerford, Oxfordshire:
William Knibbs Head Mar Ag Lab 29 Oxon, Swerford
Emily Knibbs Wife Mar 23 Wilts Salisbury
Caroline Knibbs Daur 4 Scholar Wilts Salisbury

Emily was from Salisbury, Wiltshire. Caroline was also born in Salisbury and registered as Caroline Harris, so presumably, William lived there prior to marrying Emily.
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1853

In 1853, William was living in one of four cottages in Swerford named "Blue Row". William lived in No.2 Blue Row.
The four cottages were occupied by John Harris, William Knibbs, James Moss and Edward Franklin. These houses were built and owned by John Powell of Swerford and were passed down to his son John Powell in 1853 when John snr. died.
William was a yearly tenant paying an annual rent of £3.


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1860

In 1860 William is still seen to be living in the same cottage in Blue Row with the adjacent cottages occupied by John Annis, William Annis and Edmund Franklin.
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1861

William also appears in the 1861 census living at Swerford, Oxfordshire with his wife Emily and three daughters.
William Knibbs Head Mar 36 Ag Lab Oxfordshire Swerford
Emily KNIBBS Wife Mar 33 Wiltshire Salisbury
Caroline Knibbs Daur 14 Wiltshire Salisbury
Mary Knibbs Daur 1 Oxfordshire Swerford
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1864

In 1864, William is listed as a Farm Labourer on his son William Henry's birth certificate as witnessed by Graham Barber.
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1871

William is listed in the 1871 census living at Buttercombe Farm, Swerford with his wife and children:
William Knibbs Head Mar 50 Ag Lab Oxon Swerford
Emily Knibbs Wife Mar 44 Wilts Salisbury
Mary Knibbs Daur 11 Oxon Swerford
William Knibbs Son 7 Oxon Swerford
Albert Knibbs Son 3 Oxon Swerford
Frank F Knibbs Grandson2 Oxon Swerford

Frank F Knibbs, the grandson, is Frank French Knibbs who was born out of wedlock to Emily's daughter Caroline.

Also, seen to be living next door at the time are Lucy E and Frederick Knibbs. Lucy E KNIBBS is Lucy Elizabeth, the wife of William's cousin Benjamin KNIBBS. Frederick is the son of Lucy Elizabeth and Benjamin. It can be seen that Benjamin died in October 1869 so Lucy was still a widow at this time.
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1873

In 1873, William is still living in a cottage in Blue Row at Swerford, then in the ownership of John Bayliss. In August 1888, it can be seen that John Bayliss sold the four cottages in Blue Row, three other cottages, several buildings and a meadow (all in Swerford) to Henry Clay, a "gent" of Swerford for the sum of £660.
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1881

The 1881 census shows William living at Village Blue Row, Swerford, Oxfordshire, England
Wm. KNIBBS Head M Male 59 Swerford, Oxford, England (Ag Lab)
Emily KNIBBS Wife M Female 55 Salisbury, Wiltshire, England
Wm.Heny. KNIBBS Son U Male 17 Swerford, Oxford, England Ag Labourer
Albert KNIBBS Son U Male 12 Swerford, Oxford, England Ag Labourer
Frank KNIBBS Grandson Male 11 Swerford, Oxford, England Ag Labourer

(Note: Frank KNIBBS was William's grandson, the illegitimate daughter of William's oldest daughter Caroline who wasn't living with them at the time of the census. She married George Hilton at some time prior in 1878 and was living at Lewisham, Kent with George Hilton and their two children Albert and Minnie).
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1891

The 1891 census shows William still living at Blue Row, Swerford, Oxfordshire with his wife and a grand-daughter:
William Knibbs Head Mar 69 Agric Labourer Oxon Swerford
Emily Knibbs Wife Mar 64 Wilts Salisbury
Minnie Hilton Grand-Daur 10 Scholar Oxon Swerford64

Minnie was the daughter of George Hilton and his wife Caroline (William and Emily's older daughter). Caroline died in 1883. George married Caroline's sister Mary Louisa in 1885 so it's quite probable that Minnie was just staying with her grandparents for a holiday.
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From the Banbury Advertiser, Thursday 04 February 1892:
Knibbs - January 26 at Swerford, William Knibbs, aged 70 years.
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From the Oxford Times, 6 February, 1892:
KNIBBS - January 26 at Swerford, William Knibbs, aged 70 years.

I suspect that it is Emily who 's death is listed in FreeBMD for March 1899 in the Lewisham district of Kent which is where we know her grandson Frank French Knibbs was living with his wife and children. From what we can see, Frank may well have been brought up by Emily.
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Frank was born out of wedlock to Emily's daughter Caroline. The 1881 census shows Frank living with his grandparents Emily and William at Swerford. There is no sign of Caroline.