Last year I had a flight booked with BA, through ba.com. The return flight was cancelled some weeks in advance and I was re-booked on Iberia, with a connection in Madrid. The connection failed and we had to overnight in Madrid (not enhanced by the fact that we were put in a ‘hostel’ in the middle of a business park with a single canteen which served assorted ‘slop’ which could be enhanced by a single glass of wine).
I wrote to BA and they immediately said I should write to Iberia as this was not a matter for BA. I replied accepting that Iberia would be liable but that, as the booking company, and given they have the same owners, BA should initiate this. They refused, so I repeated my request but, this time, I copied in the head of customer relations into the email. That worked and Iberia contacted me to provide details.
I wrote to Iberia and got their standard reply that this was not covered by EU261. I replied and said it most definitely was and threatened legal action. Again a brush off, so I re-submitted my email, this time copied to the head of customer relations of Iberia.
The next day I was phoned up by Iberia to say they had accepted my claim and a payment would be made to my bank account. 10 days later the full payment was received.
So the moral, is bypass the process and copy in someone senior in the respective organisations and it seems to get things moving.