Mark,
We too have a mortgage with caja murcia bank.
Basically this is how it works, if say you completed & started a mortgage in Jan, you will pay the euribor rate + typically 1%, this is fixed for 1 year, then after a year the new rate you will pay in our case is the average euribor rate + 1% 2 months prior to Jan, this then is fixed for a year ,this is repeated until the repayment of the mortgage.
There may be variations of this but thats the case with our mortgage , we have just gone from paying 5% to our new rate of 2.224 % a difference of € 200 per month. GREAT.
Hope this helps
Dave
www.rodagolfdreams.co.uk