Shame on me for doubting Aero Mexico.. After 3 months with BA radio silence and then a "no", I emailed an AeroMexico address I randomly found. 3 hours later and they have just come out and offered me 250USD.

I have to say I was not expecting that level of service and in future I won't make assumptions that's they'll be hard to deal with. I will, however, make assumptions that BA are. Not withstanding the fact they eventually offered the right advice, 3 months is way too long to wait.