A quick update for those who have helped me with this, thanks Millmoor, isobelw (apologies for missing you in my previous thanks) and ev.
I couldn't resist; I had to buy more credits on Scotland's People! And it looks like there are two James KERRs and that the naming of their children was coincidental.
The death registration for Marion Heriot KERR b. 1884 d. 1885 seems to indicate that both James and Alexandrina were living at 33 East Crosscauseway in 1885, as no other address is on the registration, and that is also where little Marion died.
I have confirmed that Alexandrina went on to marry Peter MENZIES in 1887.
While Alexandrina and her James were living at 33 East Crosscauseway in 1885, 'my' James KERR and Margaret HERRIOT were living at 26 Glover Street, Leith; at least, that is the address of both parties on their marriage registration of 1884.
Alexandrina's parents were Thomas and Elizabeth HERIOT (formerly CUNNINGHAM); Margaret HERRIOT's parents were George and Marion HERRIOT (formerly LAWSON). Many thanks for helping me clear this in my head, isobelw.
The thing is, I guess without birth dates of the parents, I can't be exactly sure, but I was willing to accept that there are two Jameses, until I compared these signatures (attached).
What do you think?