Living as a resident in Spain, but receiving pension from the UK and thus making use of the Spanish National Health System by having S1 accredited by INSS?
You will need the EHIC for medical cover outside of Spain.
This link will give you info on how to apply/renew.
Once you´re in the system as pensioner living in Spain, there´s no more need to fill out any forms, you can renew by calling 0044 1912181999. (+44) 191 279 0575 or email nhsbsa.ehicenquiries@nhsbsa.uk You have to give the number of your card which begins with UK & then the number, confirm date of birth, & address. New card will be sent by mail.
UPDATE 2015 UK State Pensioners
UK state pensioners will no longer need to produce a EHIC (for UK only. EHIC still needed for other EU countries). Since April 2015 UK state pensioners with an S1 registered in an EEA state are entitled to access healthcare in the UK as though they were normally resident using a copy of their S1.
Information on the link for applying or renewing for family members.
As a result of this concession ( which is fairly common in other EU states) the UK receives a discount of 5% on the amount that they pay to Spain. So whereas previously they would use the EHIC to reclaim the cost from Spain, this is now no longer the case.