Am I right in thinking that the booking under the "BA locator" is there more for BA's benefit, and has no (e)ticket (number) linked to it - hence why one is unable to check in using this reference?

I base this on my experiences of booking BA/QF codeshares which end up with two nearly sequential locator references, with the codeshare flight being the "real" booking, and the metal flight being the "dummy" - which seems to exist only to tell the operating airline that your bum has been allocated a seat.

Cue much seat-reservation fun when trying to reserve seats on both PNRs (possible, but the seats booked on the "other" PNR show as already taken)...