Witnesses to the marriage were Francis and Mary Knibbs, probably Sarah's brother and sister.

Sarah and John were both listed as of the Deddington parish.
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The London Evening Post from Saturday, December 24, 1763 reports the following:

On Tuesday last was married at Deddington, in the county of Oxfordshire, the Rev. Mr. John Henchman, vicar of the same parish, to the celebrated Miss Sally Knibbs, niece to the lady of Sir Harris Fowler, Bart, an accomplished young lady with a considerable fortune, and endowed with many peculiar qualifications to render the marriage state happy.

Also, from St. James's Chronicle or the British Evening Post, Tuesday, December 27, 1763:

Tuesday at Deddington, in Oxfordshire, the Rev. Mr. John Henchman, vicar of that place, to Miss Sally Knibbs, Niece to the Lafdy of Sir Harris Fowler, Bart.

Unfortunately I've been unable to find details of when John Henchman was born, but clearly, he was quite a lot older than Sarah. He was first instituted into the Church of St Peter and St Paul at Deddington in 1752 when Sarah was just 8 years old.
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I understand from reading the book 'A History of Deddington, Oxfordshire' by H M Colvin that:
John Henchman was the rector of the Church of St Peter and St Paul at Deddington, Oxfordshire, England from 1752 until his death in 1790. The book indicates that he had a very difficult job. There was a strong element of dissent in Deddington at that time with around 40-50 Presbyterians and up to 10 Quakers. Neither sect had a meeting-house and in 1771 some of them were stated to attend morning service in the parish church. The Reverend Richard Short who was the rector immediately before John Henchman wrote to the patron of the Church that he was "very sensible of the great charge I have under me & what a stubborn, factious, rude and prophane People I have to manage".

John Henchman continued to suffer difficulties and in desperation brought action in the Archdeacon's Court in 1780. A man named John Hopcraft and others were accused and found guilty of repeatedly interrupting divine service "by the great noise he and the other singers in the gallery made with bassoons and other musical instruments".

No sign in the Deddington PR of Sarah's burial.

Russell Martin told me on 7th August 2003:
"Sarah married Richard Hewitt, an underwriter at Lloyd's, and she died in London in 1817. One of her sons, Henry Hewitt, emigrated in 1827 to the Cape, settling in Cape Town, where his descendants (myself included) still live. I possess her father, the Rev. John Henchman's, Book of Common Prayer.
From this great familial and geographical distance, I send my good wishes.
Yours sincerely
Russell Martin"

Russell also gave me more information about Sarah Henchman and the Henchman family, from the researches of his great-grandfather, the Rev. James Hewitt, who was a Cape clergyman and enthusiastic Victorian genealogist. From all accounts Sarah Henchman was enormously proud of her lineage: her great-great-grandfather was Humphrey Henchman, Bishop of London in the 1660s; and through her grandfather's wife, Anne Wood, she could trace descent from Archbishop Henry Chichele. According to a family legend, she is said to have remarked disparagingly about the introduction of some improvement in the postal service (not the penny post, for that came later), "It will only induce fools to scribble." Her death notice in The Times reads: "On the 3d inst. [August 1817], at Canonbury, aged 49, Sarah, widow of the late Richard Hewitt, Esq. of Muswell-hill, and daughter of the late Rev. John Henchman, of Deddington, Oxfordshire." I understand John Henchman had been a Fellow of All Souls, Oxford. His Book of Common Prayer is autographed with the date 1745.

Sarah and Richard had seven sons, one of whom was Russell's ancestor, Henry Hewitt, and two daughters. There is no doubt more information about them in the Hewitt family papers, which now form part of the Church of the Province of South Africa [Anglican Church] archives at the University of the Witawatersrand in Johannesburg.