The correct thread for this discussion is the main EC261 thread. It is a bit difficult to tell since in your first post you gave flight numbers for IAD - LHR but in the text you said you were travelling LHR to IAD. If what I think happened was that you left earlier than expected and thus arrived early, then there is a logic to paying £260, potentially, since usually you have to be 4 hours late to get £520 or 600€ (if you started from inside the EU). There is the Azurair case, explained in post 8 to complicated matters. If you could give the exact scheduled arrival time and the exact time you arrived then I can give better advice. If it is an Azurair case then you will need to go to CEDR, but they don't always agree to Azurair, MCOL probably would be a surer bet if Azurair is involved.