Inheritance tax (or CAT) must be paid within four months of the valuation date otherwise interest is charged. The valuation date in most cases would be the date a Grant of Probate or Administration issues. So if the Grant has already issued and the property is not sold yet then you might have to pay the tax now to prevent interest accruing. There may be the option of paying the tax in installments so that you don't have to pay it all now. If you leave the tax until after the property is sold then you may have to pay interest.
I would like to come in on this, if a property is left to 2 people and 1 of them is living in the property and it is his only home when does the tax have to be paid with in four months, or when the property is sold