Hi - although Baz1946 has made avery sensible suggestion, in my experience the banks are not interested in logical arguments, and certainly not in overpaying later. They will often charge an early repayment fee for this.
There is very little flexibility in Spanish mortgages and if you do go onto an interest only one it can often mean going to the Notary and making a completely new mortgage arrangement. I know Bankia charge about 2,000€ for this and they only offer it for a limited time, and then put you onto a higher rate of interest afterwards. They are not into helping people through difficult times! Remortgaging with a different bank is also difficult and expensive.
I agree that a mortgage broker would be your best bet, and Smiley (Patrick Robinson) has offered great advice here before, but do make sure you know all the terms and conditions before making any decisions.
Good luck!