Kavanagh, the Financial Conduct Authority (FCA) have absolutely no jurisdiction in this matter. The alleged misdemeanor happened in Spain which is a bit outside their territorial limits.
Even if they were interested, the fact is that the OP will struggle to say what he's complaining about. He hasn't vaguely been through the right procedure. He hasn't even spoken to La Caixa himself and has nothing of any interest to a regulatory authority.
Kavanagh, if say you walked into the representative office of Barclays in Madrid and complained to the the largely Spanish staff of a problem with Barclays in Milton Keynes do you really think they would be remotely interested? Do you really expect them to do anything to assist? Is there a difference?
The OP has made very little progress so far and your advice is not at all helpful.