The reason for a Spanish will is that, under Private International Law, realty is governed by the laws of the country it is in. If there were no Spanish will, the property in Spain would pass under the Spanish law of intestacy, with one, it will pass in accordance with that will, subject to any Spanish law directing provision for a widow or children. In the event of no will being found and no secondary evidence of its terms being admissible under Spanish law, it seems that the property would have to be treated as passing on intestacy .
It should be fairly easy to find a Spanish lawyer, or a lawyer in the UK who is qualified in Spanish law, to advise. The Spanish Embassy is an obvious place to start an enquiry, though, in the past I have found solicitors here who were so qualified in other foreign jurisdictions simply by searching in a library. No doubt googling might find one.