Calculating inheritance tax on a house left in a will 20 years ago

My uncle has offered the gift of a house to me which was left to him 20 years ago by my grandfather when he died, however my uncle never took out ownership of the house.

My question relates to the inheritance tax which my uncle should have paid when he received the house from my grandfather and obviously has to be sorted out before he gifts me the house. How is the inheritance tax calculated?

Is it calculated using the value of the house when my grandfather died or is it calculated at the current value of the house?

Further information is available on an earlier thread http://www.askaboutmoney.com/showthread.php?t=73792.

You really need to talk to a solicitor about the most tax efficient way of dealing with this which might not necessarily be by way of taking out a grant to your grandfathers estate.