So I sent a Subject Access request as suggested. Sadly a day after doing so, I found Avios reward flights for dates I needed to HKG on Qatar’s site - went to transfer the Avios over so I could book and stumbled across the same issue!

Rang BA pleading for assistance and the best the chap could do was send a form asking for my OTP number to be changed saying it would take a week to complete. A week later, a tried expecting no change yet sure enough, it’d been fixed. Seems BA might finally be aware of this issue so I’d suggest anyone experiencing the same, give them a call …

Good luck!