Hello DeeBee,
Firstly sorry for the loss of your Family Members.
The Husband & Wife that owned the Gran Canaria apartment are classed as UK Tax Resident and not Spain Resident.
As 6 months has passed since the lady passed away there will be fines to now pay to Spain Tax Office as Spain Probate has not completed since her passing September 2013. (within 6 months)
As the Father in Law died during December 2016 you will have to Probate in Spain by June 2017 so not to receive fines from Spain Tax office on his passing too.
Double Probate is now required for the apartment in Gran Canaria where you can use either Spanish Inheritance Tax rates of Madrid for a Non Resident, or those of The Canary Islands which is the Region Gran Canaria is located.
A full review of both Spain & UK Wills is required to move forward in Spain in the most Tax efficient way.