Or if you want to do this faster and more efficiently (for you), simply make your request to the carrier for a refund and wait seven days. If, on the 8th day the carrier has not initiated a refund, file a chargeback with your credit card issuer (bank) and supply the documents which back it up at the time of the chargeback:
1. Copy of EC 261/2004, Section 8 - highlighting the refund as an option. (don't supply the entire document).
2. Copy of e-ticket receipt.
3. Copy of notice of cancellation (or significant flight time change).
4. Your request for a refund.
5. Carrier response (if there has been one) or note that you have not heard back,
For UK and US banks, a temporary credit should appear in short order and that will become permanent unless the carrier can somehow demonstrate that it did not cancel (or substantially change) the flight.
No need to send letters before action, pay filing fees or do anything beyond the above, Most issuers have an online portal for this, including the ability to upload the relevant documents. For others, it may cost you a stamp. Don't be tempted to do this orally, even though it may save you a day or two.