Kippen,
You use a different search strategy to me. For an event I search all possibilities in a particular area in a particular era. If successful, I spread out geographically and extend my time window.
You say that you have searched ""ALL cross border marriages in the Berwick area"... "but in BOTH places"
Did you browse through the Coldstream Bridge records.
You are right that if a marriage took place (in England) after July 1st 1837, it should have been registered in/with the appropriate Register Office (e.g. Berwick for Berwick and Tweedmouth area, Belford for North Sunderland area)
and show up in GRO(England/Wales)
Civil Registrations of BMD in Scotland did not start until 1855.
Although some English folk who crossed into Scotland to marry did use the various "Scotch Priests" to give them the bare bones of a marriage, others married in churches of their particular religious denomination.
A small hint from me... do not plan your search strategy on your "oral history"... Oral histories can include errors and inaccuracies.
Am I right that Alexander/Sarah only had one child born in Berwick-upon-Tweed (William 1849)... the others been born Tweedmouth and North Sunderland.
From censuses 1851,61 it would appear that Joan/Joanna was their first child ( b.c1845). But I notice the birth registration of a William in Sept 1843, followed by a death registration in Sept 1844. ( The English/Scottish Naming Patterns would have "required the couple to name their first son, after the child's father's father , which I think was William ! )
It would appear that Alex/Sarah followed the law of the land and fastidiously registered the births of their children (first eight at least). This doing of their duty would allow me to have the hunch that they also baptised their children in some church(es) !
Did you know Sarah's maiden name (Johnston) from the middle name of her child Henry ( although Stephen's second name seemed to have been Johnson).
I noticed on Census 1841 in Tweedmouth, there were two Alexander Grahams... both bakers, one 17, one 20 yrs.
Michael Dixon