inheritance tax.

Myself & my wife are 81 & 80. We own a Spanish property in the Valencia province worth approx. 250000-275000 euros with a little cash in an English offshore account. We have made Spanish & English mirror wills, in favour of our son age 50 & grandaughter age 13. We are not full Spanish tax residents & our Gestor submits our details each year & settles our tax for which we supply the funds.
Could you please advise as to the amount of inheritance tax that would be liable when one of us & then the survivor dies.
Regards,
A. Islip