Hi Jilly1,
I think you might have problems on finding a site that will give you the information you require, most as you say just show holiday or short term lets. One of the things you could do is email some of the sites where you see a let that you like and say that you are interested in hireing that property for a minimum of 1 year, I think you will soon get them coming back to you.
I can only talk about the Costa Brave which is N.E Spain in the provency of Catalunia. The 2 main cities you will know are Girona and Barcelona.
I live about 35 minutes from Girona in a vilage called Sant Antoni de Calonge, and 5 minutes walk form the sea but away form the madding crowds as they say.
Just down the road from us is a very plush up market resort called Palamos, which is open and active all year with style glamour and high class shops. It is very busy during the summer season which runs from about 1st June till the end of September, but after that all is back to normal as they say. I you want the peace and quite then you would not want to live directly in the town, but there are many peaceful and quite areas about 1 mile out of town.
I take it from your comments that you intend to come and live in Spain when you retire, and the property you are looking for is to rent out so you have some income until you can move in yourself. I hope I have got this part right, if not please advise what your intentions are if I am not being to rude.
I hope this little information is of help.
Regards
a1gjv