Ok didnt know about the John of 1776, our John was defo born around 1790ish according to the 1871 census which puts him at 80yrs. So hes probably the John later baptised in 1794.
Ive looked for deaths of children and second marriages but cant find a thing. Isabel Burn was the name of the wife in Elsdon 1779 (I think!)
Its seems obvious that there was a lot of Murton or Morton activity in and around Morpeth and Newcastle at that time, its just a bit of a mess at working out which set is ours. Annoyingly John married his wife Ann around 1833 so before certificates and therefore no father details.