If you are residing in Spain, you should become a resident, and therefore pay taxes as a Spaniard does.
Any income, pension, benefit should be declared.
This year the H.M.R.C has informed the Spanish tax authorities of U.K. Citizens, receiving all kinds of income. I know this is true because my fiscal advisor
showed and read out information, that was on tax form
here, just a few weeks ago.
So I would advise people to seek advice and help to fill out a tax declaration form here.