An extremely capable BA agent got her teeth into this, found my reservation through the flight details, re-entered my BAEC number, clicked away for some time then gave me the seat i wanted and a BA PNR.

She thinks there was something wrong with the way the BAEC number was originally captured, so recommends getting it re-entered at each check-in.

We drew a veil over the effort made by her colleague I'd previously badgered about this.