John, you are a scholar and a gentleman.
The BIG tricky one is William Armstrong b about 1791 in Ravensden, (NOT Upper Gravenhurst) but I guess you want to look him up for yourself anyway!
He doesn't apear in the Ravensden batch on the IGI as a child of John Armstrong and Sarah, but that is not necessarily meaningful, as nor does a Sarah born about 1807 who died in 1810 aged 3 according to the NBI.
Then there is the marriage of John Armstrong Snr b about 1760 and Sarah - but as we don't have a clue where they came from, that's a hard one!
However, Wilhampstead might be a possibility. John Armstrong 1760 turned up in Ravensden in 1787 and Thomas Armstrong 1756 who married Phoebe turned up in Upper Gravensden in or about 1788. There is an IGI entry that places Thomas's birth in UG, but it's a member entry based on his marriage to Phoebe which simply assumes he was born where he married.
John Armstrong and Edy Grange of Houghton Conquest married in HC in 1753. William Armstrong was baptised in Wilhampstead in 1754, son of John and Edith. John Armstrong was baptised in 1761, son of John and Edith. Might they also have had a Thomas b 1756 - he would fit nicely into the gap!
There is a Thomas, but he's baptised in 1766 son of Thomas and Christian Fiddes, so 10 years too young.
John b 1760 named two of his children John and Edith, which is a slim link, but nonetheless it IS a link!
It might be worth checking Wilhamstead and/or Houghton Conquest, to see if there is anything in the PRs which isn't on the IGI.
If you can find anything, I shall be very grateful - this is my direct line, and I am thoroughly stuck.
Best regards
Wendy
PS Another bit of circumstantial stuff - the Chandlers who joined up with the Armstrongs when Elizabeth married George in 1857 were pretty strict Primitive Methodists. They too baptised their children in batches. I wonder if the Armstrongs were too? Maybe their patchy baptism record is simply because they had to wait for a Methodist minister to pass by? And if one didn't, then they just didn't bother with baptism? I have something very similar in Kent, with a Catholic family living in a Protestant parish.