Does Plus Valia Tax have to be paid in addition to Inheritance tax when, as a result of a will, it is transferred and not sold ?

I have just been asked by my lawyer to pay E4500 on top of E500 already paid for Plus Valia Tax. I have no idea how it is calculated - can this be checked
My Husband died in 2006 and left his 50% of the property in Puerto Banus equally to our 3 children. The value at the time of death was approx E300,000 on an original purchase price of approx E100,000.

Regards Jacky