Hi.
Can you identify the reasons why there is a difference in valuation for similar apts in the complex Nick? Things like access to pools, orientation and views can make valuations different for example (im sure you know these already?).
Can you approach it from the point of view of a mortgage and ask for a professional valuation based on a loan perhaps.
The easy way to look at a valuation is what someone is prepared to pay and the other is prepared to accept, but its more complex than that when there are other comparables. your just starting the journey but please read current posts about agents and valuations and reposessions in Spain. It seems to be a minefield that you should be very careful to tread through.