Unfortunately, it is the lawyer who dealt with the sale in 2011 who is not releasing all the information being requested.
Perhaps you should consider reporting this lawyer to the Murcia Law Association. Every piece of paper having to do with with a case belongs to the client, and a lawyer can never refuse to hand over a file.

However, if you still wish to request a copy of the documents, the tax office that covers La Manga region, is the one at Calle Campos 2, Cartagena.

Regarding obtaining copies of the deeds, the first step would be requesting a 'nota simple' from the land registry (around €9). This document will show the current owner and the name of the notary public who witnessed the signing of the deed. You then look it up on the Notary Association website, and you can contact him/her to request a copy of the deed.

Should you need help with this, Juan Carretero from Lawbird should be able to assist you for a reasonable fee.

Regards