Writing, letter to the Hacienda is not going to get you very far. As they will not reply. Another thing that the British have not learnt. Besides writing in English is not going help your cause. An on line translation of your text is not going to help either as the style of business letter writing are very different.
The Spaniards solve this problem buy going there themselves. You need to go there. If you don’t get the address of the Hacienda by the forum user, go to the Notary office, where the sale took place and they should be able to point you in the right direction.