Hi Ruth
When you get your work contract you take that along with all the usual paperwork for you and your dependants so that means NIE and residents certificates passports and copies for children you need birth certificates spouses you also need marriage certificates the forms you can obtain here on the website for Social Security
www.seg-social.es/Internet_6/index.htm
The office addresses are as follows www.seg-social.es/Internet_1/Oficinas/Listado/index.htm
I believe you will need to go to either the Orihuela or one of the Elche offices but it may pay to just check this out before you start out or ask on perhaps one of the Torrevieja forums www.torrevieja.co.uk is a good one and many people on there are working and should know which office from your area.
Nº8
Avda. Alicante nº 2
03203 ELCHE - ELX(ALICANTE )
Teléfono(s): 96 5454875
Nº de Fax: 96 5420753
alicante.administracion8@tgss.seg-social.es
Plano de Situación
Nº9
C/ Obispo Rocamora, 43
03300 ORIHUELA(ALICANTE )
Teléfono(s): 96 6736702
Nº de Fax: 96 6737442
alicante.administracion.9@tgss.seg-social.es
Plano de Situación
Once you have the affiliation form stamped by this office you can go to your local centro salud for to apply for your permanent SIP card.
Whilst you are there you can request the reciprocal version of the European Health card which covers you if you return to the UK for visits
Good luck and please let me know how you get on...