Eurozone transfer - Intermediary bank charge?

I make semi-regular international transfers to a bank account in Belgium using Bank of Ireland online banking. Previously, these transfers have been free of charge, as BOI claim they should be.
However, €15 was deducted from my most recent transfer before it arrived at my Belgian account.
I raised this immediately with BOI who assured me that they don't deduct any charges, and suggested that it could have been the destination bank's correspondant bank who did this, and suggested contacting my Belgian bank to resolve this. (I'm now pretty sure that the correspondant bank is BOI's, not my Belgian bank's)

Eventually, they got back to me today and informed me that it was the correspondant bank of BOI in Belgium who deducted the €15, and that I should contact BOI.
I've done this today, but have to wait at least 3 days for them to get back to me. To be honest though, I'm not optimistic and I'm expecting BOI to come back to me and say that its not their fault.

Does anyone have any suggestions? Do I have any right to free transfers within the eurozone? Do I have any comeback against BOI who claim to facilitate free transfers? Has anyone had this problem before with BOI or another Irish bank?

What I find really strange is that this is the only time I've suffered any charge.

Regulation (EC) No 2560/2001 of the European Parliament and of the Council
of 19 December 2001 on cross-border payments in euro provides thatcharges levied by a financial institution for cross-border electronic payments to 50,000 EUR are the same as the charges levied by that institution in respect of corresponding payments in euro in that Member State. You must provide the beneficiary’s BIC and IBAN nos for this to work. So, did you provide the BIC and IBAN nos and was the transaction in euro and for less than 50,000 EUR?

Thanks for that information, very encouraging. Yes, I provided the necessary codes, BOI's online banking requires that you add these when setting up the beneficiary, and the amount was also less than EUR 50K. I'll wait for them to back to me so.