Probably not of interest at this stage, although it might help to trace his early career and occupation, but I think I have found Sarah's baptism, given her age on the second marriage.
11th February 1771 at Evershot, Dorset, daughter of John and Martha. Approximately 8 miles from East Coker, where John married Martha.
Also Mary on 12th May 1773, born 9th November 1772, and John on 11th February 1776.
At Cerne Abbas, Robert Chaffey Packer and Elizabeth Packer on 22nd August 1785. He is on the 1851 census in East Coker.
What might help, although an expensive gamble, is that Robert, widower, married again in 1847 to Mary Crode in Netherbury, Dorset. It confirms that his father was John but, unfortunately, only a transcription. The certificate might help to clarify his father's occupation. His former wife, Phillis died in 1842.
It might be worth a post on the Dorset board, in case anyone can view the image.
Memorial inscription East Coker:
Coped Stone, flaking and falling apart, nothing legible) Robert PACKER, born Jan: 9, 1784, died Feb -- 1855 (PR: buried 17 Feb 1855, aged 75). Phillis Packer, born Jan: 13, 178-, died March 22, 1842 (PR: aged 60, i.e. born Jan 13, 1782)
David