Thanks for the advice! This morning I've checked the AA PNR again and it seems my last call to BA did the trick as AA have finally removed the sector on AA100, and checking the seat map through the seat alerts app I can see the seats we had allocated on that flight have now been freed up - great! BA MMB still shows the old seat assignments against the BA1517 sector but I'm not as concerned about that, maybe it'll pick up the correction in a few days? Or it simply doesn't matter as I guess it's the underlying AA reservation that's important for this sector.