Two points, David:
"Maulden parish register
OVERSEERS of the Poor, Apprenticeship
FILE - Bond (£20) Wm Batterson of Cople - Cordwainer, to Maulden Church Wardens re apprenticeship of Wm Armstrong. - ref. P31/14/53 - date: 12 Mar 1766
It's possible that the William who married in Cople was from Maulden"
Again, supposition, but it fits.
John and Edith were in Wilstead till 1760-ish when John was born. They appear to have both died in Maulden in 1773 and 1770 respectively. Which would have left two orphans: William aged 19 and John aged 13 when their father died.
And an interesting point about adult baptisms - William the Wesleyan and Mary baptised their children in Wilstead more or less as they were born until 1808, when they introduced Methodism to the village. They then had at least three more children, Joshua in 1808, Thomas in 1812, and Elizabeth in 1817, who were baptised in a job lot in 1829. All are on the IGI, and the batch includes all the children of John, their eldest son, as well.
I haven't got as far as A2A for the Armstrongs yet - still poring over the IGI and the NBI!