We always use beninter. They tend to be private apartments in residential blocks.
I can recommend Gemelos XX and Torre Levante
http://www.beninter.com

I'd be careful about booking without using the well known local agents like Beninter, Gestaltur, Fincas Arena etc. OK you might pay a few quid extra but at least there should be an apartment for you when you get into town. There are a few cowboys out in internet land!

I've booked the Gemelos 22 through Travel republic, 2 adults & 2 kids for £369 for 2wks.

Try Viajes alameda they are a Spanish rental company that I can highly recommend for appartments and transfers.