I am a resident here and receive the basic UK State Pension (paid into the UK). This is my only income. Do I have to pay tax in Spain on this?
If you are resident here you must declare it in Spain. You can then opt to not declare it in the UK. You can get a certificate to send to the Inland Revenue to state you are declaring it in Spain. A gestor should be able to sort this for you as part of your annual tax return. UK and Span have a reciprocal agreement and share information. If you don't declare it in Spain and they catch up with you there will be fines to pay.