Good luck. This happened once to us on a flight from JNB-LHR. Seats were blocked and unavailable so our original booking was changed to separate seats, though we did request again both at check in and boarding to be seated together. They upgraded someone to the "unavailable" seat at the last minute. My spouse reported that the seat in question was fully functional, so maybe it was fixed? This happened again on the ongoing leg from LHR to SEA so I did submit a complaint to BA,but not a whisper of a reply.