Hi Jarvi,
It does seem very odd.
In my experience the premium for Mortgage Protection Insurance should be static - I've never heard of one that increases.
If you have a repayment mortgage (ie where the amount owed gradually decreases) this is even more absurd.
If the mortgage is "interest only" it makes a bit more sense, but it's not the way it's done in the UK.
I suggest you have a close look at the mortgage quotation and offer letter they will have given you. Did they state the premium, implying that it was static, or was there any indication that it would increase? To me that is the starting point. Then possibly speak to another bank and obtain a quote from them and ask the question as to whether or not their premium increases. To me this is weird and illogical, but it would be good to know if it is customary in Spain, the two are not mutually exclusive.
I would probably be more demanding than Claire T, but it does no harm to try and settle it amicably first.