I get off the phone with BA, after they confirm there is an A seat available to them, quickly call CX, who say they can't see it, so they drop that sector and re-enter it, but still no good, back on waitlist.

I called BA back, and the seat is still there, so it's not as if someone else took it.

The only thing Cathay could think of is that BA have separated allocation, so that BA will sell the seat as A, but not offer to other parties.

I'll just have to wait and hope. Searching through previous threads shows a potential reason - Point of Sale