There is a possibility that I may be able to do some temporary work via an employment agency on an "as and when required" basis, working full time for just a few weeks in the year, maybe one or two days in other weeks, and not at all for several weeks at a time.

Could anyone advise me what the position would be regarding the social security system for agency workers with this type of arrangement? I certainly wouldn't want to have to register as autonomo when I wouldn't have any income at all in some months, and I don't need state health cover or to build up pension rights in Spain.

I'll be watching any replies with interest, with the add-on of freelance, as & when, not full-time but project-based!