THIS WAS POSTED SOMEWHERE ELSE BUT MAYBE OF INTEREST.

I am new to this forum and found it very interesting, specially this thread.
We are planning to move to Spain in 2007. I have been having discussions with my employer to allow me to work remotely from Spain as I am home based in the UK and all I need is a broadband connection. I just came back from Spain and researched the issue thoroughly with my Spanish solicitor. If you work for a UK company, you will, be paying UK taxes. You will not be a Spanish resident even if you leave there more than 6 months in a year. You will not need to fill any tax returns because the taxation agreement between the Inland Revenue and the Hacienda. There is no need to pay Seguridad Social (NI) in Spain because you are paying in the UK. Providing you have the new European Health Card, you will be covered if you fall ill.
I am not sure how all this will apply if your employer does not know that you are base in Spain. I would be careful if I was you as leaving the country and working from abroad may constitute a bridge of your employment contract and you might end up in Spain without a job!