Johnx: Please let me what is confusing in the post so I can happily clarify ;)
Floella: Please have my comments/ answers below in bold green:
So to clarify the situation, as I understand it, according to old ruling when a Brit, whose only estate assets are in Spain, or better said: if he does not have estate assets in the UK dies the UK will have to refer back to Spain and the inheritance laws therein. Consequently any family member not included can challenge the will, under the rules of Spanish Law, and Spain receive all inheritance taxes. Spain will receive inheritance taxes related to the property regardless National law regulating the sucesion, as this tax applies by REAL obligation ( situation of the assets) and not by PERSONAL obligation ( residency of taxpayer)
However rewriting ones will in accordance with the new ruling from Brussels, will grantor can choose between Natinality or Residency Law, despite UK has not signed that agreement, as UK forward some inheritances to Spain, Spain Inheritance Law will have from August 2015 on, this European Regulation as a superior-international regulation of the matter, so the UK national will be able to choose back his national law. There might be disputes in the interpretation of these new regulations by different Courts in and our Spain and UK. So the most preventive measures you take in favour of what you decide/desire, the better. lawyers can dispense with the deceased estate as is recorded in the will. Unfortunately it will not stop any non inheritor challenging the will as this a common practice in UK and Spain will still receive all inheritance taxes.
Again: Spain will receive all inheritance taxes regardless Nationality of Inheritance Law being applied, for reasons explained above.