OK, thanks. You can sue them for the e328 under EU consumer law.
You should send the airline a letter before action to say that you believe your consumer rights have been infringed and that you demand repayment of the further payment within 14 days, failing which you will lodge a small claim at the county court business centre (which then gets sent to the airline's head office address).
You can use the European Small Claims procedure in order to claim. This allows for cross border claims of up to 5,000 euro in value. Assuming they do not respond or they do and refuse to pay, you need to fill in the attached Form A and attach any evidence you wish to rely upon to your local civil court.
The form asks for your bank details and the 25 euro issue fee will be deducted but added to your claim sum.
The court will then serve the form on the airline who then has 30 days to respond. The court will consider the response and give judgment on the claim (or if further details are required the court will write to either party). They may hold an oral hearing (by telephone if the court has the facilities - which most do) but with the facts of this case it is unlikely as it should be capable of being dealt with on paper. If you win you get a certificate against the airline which means they must pay your claim.
With the letter before action, you can email it to the airline and include your bank details so they can pay you (ensure you give them the IBAN and BIC code, name of your account and name of the bank).
The European small claims procedure is a very efficient and quick way to obtain a court judgment so I encourage you to pursue it.
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