You will have to file an initial tax return to get on the system. With £1100 between you then you probably won't pay any tax or, at least, very little. You get an allowance (like UK) plus other allowances such as married, children, mortgage and so on and then another allowance on top depending on how much you earn.
Once on the system you will receive free healthcare.
However, are you also getting an income? If so, you will also have to submit a tax return and you need to work out if a joint return or 2 single returns are more advantageous (different levels of allowances).
If not getting an income, then I would think very seriously about moving to Spain. Once rent has been paid, that doesn't really leave a lot for day to day living. I would think a couple could live happily on £1000 a month but only after rent has been paid. Remember, you electric bill will be almost €100 a month to take off that.