Before I went freelance I was working full time for a company for over 3 years as autonomo and I was based in their head office using their equipment. But i did have a contract of my services with them rather than a contract of employment. So maybe that is the answer... have a contract drawn up to say that you are providing them a service.
Well I dont really know much on the subject but thats a little bit of my experience.
I think most employers are reluctant to offer a contract of employment cos with Spanish laws it is hard to sack someone on a contract of employment. I heard about someone on a contract being asked to leave the company but he refused as he was on a contract. So in the end they had to pay him money to leave, and I think he done quite well out of it... but dont know how true that is.
Paul
http://www.paulscottondesign.com