Another will (sorry!)
Will of Mary Dutnell (Durtnall) of Lewes, The Beechwood, St John under the Castle, widow
written 1767, proved 1776, Archdeaconry of Lewes.
Could be an important one? Quick look, names brother/nephew Thomas Siffleet (and more of them, even Mary Soper!!)
two pages of it here
https://www.familysearch.org/ark:/61903/3:1:S3HY-D189-XV2?i=614&cat=685691This was after a trail back via The Keep
13 Dec 1806
Letters of Administration of the goods of Elizabeth Killingbeck left unadministered by Mary Killingbeck granted to Josiah Siffleet of Hamsey, gardener, cousin germain and next of kin of said Elizabeth.
There is more related stuff from the same source on The Keep website.
Anyway, it all starts (I think!) with a marriage
27 Feb 1714/5, apparently at Maresfield
Thomas Killingbeck + Mary Siffleet
It was by licence (Deanery of South Malling)
The marriage licence register says both of Chiltington
https://www.familysearch.org/ark:/61903/3:1:S3HT-DT69-KB5?i=1152&cat=604182Thomas and Mary had a (only?) daughter Elizabeth. Who may have been baptized as an adult? Elizabeth must have died circa 1753, because
4 May 1753
Letters of Administration granted out of the Archdeaconry Court of Lewes to Mary Killingbeck, mother of Elizabeth Killingbeck late of Beechwood, deceased, of the estate of the said Elizabeth
The widowed mother Mary then married again
10 May 1753
Assignment for £100 by Mary Killingbeck, widow, with concurrence of Thomas Dutnell, to William Michell of Lewes, gent, in consideration of intended marriage betwen said Mary and Thomas.
Family Search has the marriage, Thomas Dutnell + Mary Killingbeck, 24 May 1753, Rodmell.
All those years later Josiah administers the estate of Elizabeth. Is he the Josiah you found who was buried in 1832 age 79?
John