It's up to the airline to prove that there were "extraordinary circumstances" which "could not have been reasonably forseeable." However, in reality that means you need to sue them.
There was a test case against SAS which suggests that mechanical failure from an airline's hub is not a satisfactory excuse.
You may want to try EUClaim.com Not sure how much they charge but it will take the head-ache out of the claim for you.
Otto