I believe this relates to the infamous Clausulas Suelo or 'Floor Clauses' (sometimes translated as 'Soil Clauses') which if included in your mortgage contract kept interest at a minimum of something around 4% and a maximum of 15%. These clauses meant that the bank always benefits while the customer never will.
The Supreme Court in Spain ruled on 9th May 2013 against certain banks and told them to remove these abusive clauses. Our interest with Cajamar, one of the banks ruled against, was reduced somewhere around August that year and we received a refund back to 9th May. However the ruling did not include anything definite about retroactivity back to the start date of the mortgage.
There are countless court cases about these clauses against banks not included in the original Supreme Court ruling. In the majority the clauses are annulled by the court. However the question of retroactivity to include the period between the start date of the mortgage and 9th May 2013 has never been clearly sorted out, and the Supreme Court recently declined to rule definitively on this point. So if your mortgage has a Floor Clause it should be a good bet to go to court to have it removed, but whether a retroactive refund is awarded will depend on which court deals with your case. Many Audiencias (provincial courts) especially in the north of Spain have ruled for retroactive refunds but some have not, including the Audiencia in Granada where we have our mortgage. So we benefited from the Supreme Court ruling 9th May 2013 but can't claim the overpayments back from before this date.
Seems to be a lottery given that the provincial courts don't seem to have to take notice of judgements elsewhere. I would recommend talking to a good lawyer who has had success with these cases. Maria may be able to help?
Plenty of articles about these cases posted on Facebook but the guys who won the landmark case re Bank Guarantees. Most in Spanish. Try the link below, or Google Floor Clauses in Spain. I just found this article
http://www.onthepulse.es/finance-business/which-spanish-banks-still-apply-floor-clauses-their-mortgages
and this is the Facebook page for Spanish Off Plan Property Bank Guarantees.
https://www.facebook.com/pages/Spanish-Off-Plan-Property-Bank-Guarantee-Abuse-LEY-5768/134524803233863?fref=ts