This realisation was particularly brought home because I found a burial record for a chap of right name, parish affiliation & occupation, but I'm not sure it's the right chap. You see I'd been looking for more children in the family - and skimmed 5 years of baptismal records from 1812 to 1816 & didn't find any. So I started looking at burials in that period & found this chap John Patterson buried in 1812 who could be the father, which would make sense of no further children. But he's listed as a widower, so that would mean that the mother (Frances) had also died in the 8 months between last child birth & his death. No burial record for her in that parish where they'd been living, so I started looking for the right years in the next-door parish where I know their son moved to & the father had come from. And by chance I stumbled across a baptism for a child of a John and Frances Patterson in that parish in 1815. So was the burial of John Patterson someone else?
I'm inclining, at the moment, towards the idea that yes it was someone else (possibly a generation above in the family but that's pure speculation based on the age alone - 50 in 1812 seemed potentially a bit old for the JP I was looking for whose first child (with Frances) was born in 1806). I've gone through the baptisms back to 1809 (from 1815) so far in this second parish, and I found one more child in 1813, but none before that. So, so far there's no overlap with baptisms in the first parish (which range from 1806-1811). If there remains no overlap I'm inclined to identify them as the same family. Particularly as the abode given for them is Philadelphia which is between Penshaw (the first parish) and Houghton le Spring (the second parish). And in the various census returns Stephen Patterson (son of John and Frances in J's direct line) gives his birthplace sometimes as Penshaw, sometimes as Shiney Row, sometimes as Philadelphia. So it ties in.
One of the things that makes it difficult is that 1812/1813 is a boundary for how the bishops' transcripts were done. 1812 and before there was no set format for the baptismal records, so each parish just did what it saw fit within particular rules. 1813 and after there was a particular required format. Unfortunately before this change in Penshaw & Houghton le Spring they provided the useful information of which child this was - so I know that our Stephen was 1st son of John & Frances, for instance. But afterwards this wasn't in the format (but instead abode was, which is also useful). It would've been nice if they'd switched where they baptised their children one child earlier in 1811 (if this is indeed the same family), as then I'd have child numbers to compare & provide one more clue as to whether or not they're the same people. Or one later so I'd have the abode to compare. Also, 1812 & earlier records seem to provide mother's maiden name, but after the change they don't, so again it'd be nice if they'd moved just that one earlier :/
So it's all guess work, but it's educated guess work at least. And I'll have that bit more confidence once I get back past the potential overlap period.