I couldn' find a baptism for Sarah in the Chinnor part of the OFH CD, in your date range, but her marriage to John Witney is there dated 26 July 1828;witnesses: Wm Witney and Ann Hester.
In passing, Edmund Keen married Ann Heyburn on the same place, same day. Their witnesses were Thomas Heybonn(!) and Joane Bennell.
The Chinnor records also have the marriage of Charles Keen to Hannah King, 12 0ct. 1828.
There were Keen, Kean, Keene and Kenna in CHinnor from the late 1500's.