On the assumption that the couple stayed in Sussex and did not return to Kent, I have looked for their deaths.
I can't say this is the right one but there was a burial of a Sarah Sophia Small in St John the Baptist, Westfield on 5th November 1836. It gave her age as 60, making a birth year circa 1776. Her burial in Westfield was followed by a John Small on 29 Sept 1839 whose age was given as 71, making a birth year of circa 1768 which (unless he had lessend his age to be closer to that of his spouse) was a bit far off the birth year for your John.
Amongst alternatives is a John Small buried in Mayfield on 24 August 1813 aged 72, giving a birth year of 1741. Of the two, Westfield may be worth further investigation.
What parish was the Thomas Henry Small listed on the website born in, for that suggests he died in 1885, but I found no trace of that death on FreeBMD, or of him on the 1881 census or on several censuses that preceded it?
Roy G