I had similar issues with other airlines LH, LX, Thai) before. I wrote a friendly email quoting reservation number, flight number, day, reason for delay and the compensation I was requesting (including scans of the receipts if any). I all cases I got an email back a few weeks later that the feel really sorry but that in this case they are not obligated to pay since (following bogus legal explanation).
I responded with another email telling them that their explanation is wrong and EC261 covers this specific situation. I would get a lawyer to take they case to court if they don't pay up until xx (around 3 weeks later). In all cases the airlines paid then.
One situation with BA was different. They paid after the first email.
Trythat first, you then still can sue or get the help of a specialized agency.