If you are are UK resident go to www.ehic.ie and apply online if you need to renew your card or to download a form. Alternatively go to your doctors surgery.
There is NO charge whichever process you use. Without this card you will be charged for any treatment you have and it will be quite a complicated process claiming your money back once back home. The card is FREE so there is no excuse not to have one.
If you live here in Spain you go to your local Social Security office. For Benidorm and surrounding areas the office is located in the Old Town on C/Jupiter - at the crossroads of C/Thomas Ortuno and the office is open from 9am until 1.30pm Monday to Friday. Be aware that only people who are legally working/ paying Social security and resident get them from Spain. British pensioners, who are resident here don't get them from Spain any more, they have to get them from the UK too.
I went up there earlier this week to renew mine and until last year it was issued there and then. Not anymore so be warned. My son is going to the UK in 2 weeks but as I was in Benidorm thought Id get it there and then - I'm glad that I didn't wait until the day before!
You get a ticket number and wait for it to come up on the screen then go to the designated desk. You will be issued a receipt and told that it will come in the post in 4-5 days .... but remember this is Spain and Correos are not known for their speed - so check if your card has expired and if so renew it now in case you need to make an unexpected trip back to the UK.
You can apparently renew it on the web too but I tried and couldn't make it out - and Im fairly PC literate. Alternatively you can call 900 166 565 - but unless you speak fluent Spanish or have an interpreter don't bother...pop up to Benidorm, renew your card and go for a coffee or cerveza.
Also remember to actually have the card on you when you go out .... if you have an accident its no use stuck in your hotel room!