Consumer Protection Agency ?

Is there a service or agency one can complain to when a store does not uphold its guarantee on a product?

 Of course yes, every Local Council has a service for Consumers Protections called OMIC ( Local Office for Consumers Information). There is generally a lawyer there who will help you through the claim.

And who do I complain to about a claim against Iberia Airlines?

I assume by 'local council' you mean the Ayuntamiento of the town where we live?

Many thanks !

 Did you already try the consumers service of Iberia itself?

Yes, I received an automatic answer that they would answer me, but never heard anything more.  This is the second problem we've had with Iberia, each time costing us between 500 - 1000 euros for their inefficiency.