I assume you were a former Abbey National account holder. There were/are many such people in the UK and Grupo Santander run an information service ato help such people sell their shares. Website with phone numbers etc is here.
http://www.abbeynational.com/csgs/Satellite?c= GSInformacion&cid=282596177748276&pagename=Abo utAbbey%2FGSInformacion%2FPAAI_generic
You don't have to use their service, but unless you are familiar with sharedealing it may be best to consider it because they will deal with the questions that have to be answered regarding Spanish tax liability (you are very unlikely to have one). Mother-in-law sold 100 shares using this service about 2 months ago - worked fine.