Originally Posted by overseasplease

I was booked JER-LHR-ZRH on 06. March. The LHR-ZRH leg, BA712 scheduled arrival at ~12:00, got cancelled due to fog at LHR. I was re-booked onto BA750 to BSL with scheduled arrival at BSL at 06:20pm. At the (slightly delayed, ~30min, due to air traffic restrictons due to the fog) departure from LHR the aircraft experienced a technical failure of its communication system during taxiing then returned to the gate for repairs and a subsequent change of flight crew as they were now going above their hours. We finally landed at BSL at 09:27pm (FlightRadar)/09:29pm (FlightAware) which constitutes a delay of 3h7m (FlightRadar)/3h9m (FlightAware).

This is basically what I sent in to BA to claim for a >3h delay EC261 claim. I also attached the subsequent apology email from BA which read: "Dear Mr Overseasplease, we're sorry for the delayed departure of your flight to Basel today. A technical issue was identified prior to departure which resulted in the aircraft returning to stand, allowing our engineers the opportunity to resolve the issue as quickly as possible. Safety is our highest priority, and we would never operate a flight unless it is safe to do so. Thank you for your patience during this time." etc etc.

BA has just now responded and refused my claim, saying the delay was due to fog-related air traffic restrictions (which were rampant on 6th March as you might recall) even though we returned to the gate, had mechanics change comms systems and they had to call in a standby flight crew. This must sureley be in their records. And, as mentioned, BA confirmed this in their email. How can I best go back to them and dispute their refusal for the claim?
Many thanks in advance for any pointers!