EasyJet ignoring EC261 compensation claim—what recourse do I have?

I submitted an EC261 compensation claim to EasyJet. They responded in a few days, saying that they need to verify the reason why the flight was cancelled, which may take up to two weeks. Three weeks later there was no response, so I responded to their email asking for an update. No response. After four weeks, I asked again. No response.

What recourse do I have in this case?

Some more details:

• The ticket was not bought by myself. I believe it is the passenger who is entitled to compensation not the original buyer of the ticket. Is this correct?

• The flight was originally delayed by multiple hours. During this time, the airport staff told passengers (who were kept at the gate for 2 hours, after checking boarding passes) several times that this is because of an engine problem (EasyJet's responsibility). There is no written record of this. After 6 hours of waiting the flight was finally cancelled due to bad weather (not their responsibility, but this wouldn't have happened without the original delay).

• The flight was from France to the UK

I would not normally pursue compensation, but in this case I was (and still am) extremely upset. I was travelling to a job interview, and I believe that the delay contributed to the outcome of the interview in the end. EasyJet handled the situation very badly. There was no EasyJet staff at the airport past the security barrier to ask for information. They only communicated in mass SMS/email. Outside the security barrier they did have a counter, but people there would not help at all, saying that I need to call customer service, which of course is a paid service and they never pick up. They refused to put me on another flight the next day, saying that I would have to book it myself after refunding the original ticket. This caused a multi-day delay in my travel.