A few months ago my flight from Zurich to London City was delayed. Due to the lateness we were diverted to Gatwick instead giving a delay of over 3 hours at London City. On weekends there are flight restrictions at London City so we were too late too land. British Airways are denying my EU261 saying the delays were due to Air Traffic Control restricted operations so is out of their control. However the initial reason for the delay was that the inbound flight was late arriving. I've asked the reason for this but so far they have ignored my question and just repeated the air traffic control restrictions. Is there anything I can do to escalate this? They have been so slow at responding I think that it's too late to try the section 75 claim via my credit card.