The Government changed the Inheritance Tax (IHT) rules some years ago now to help people more easily plan to pass on their estate and avoid IHT. There is more information here: http://www.hmrc.gov.uk/inheritancetax/intro/transfer-threshold.htm
The main change was that where previously each individual had to take advantage of their own nil rate band (the amount that can be passed on free of IHT), now married couples are allowed to use up the unused allowance of a previously deceased spouse. That means married couples are no longer at the mercy of advanced IHT planning in order to be able to use up both allowances.
However, one slight confusion remains. What if your spouse passed away before the IHT rules were changed?
Essentially this does not matter. HMRC allows a person that passes away now to also use up the unused allowance of a married partner that passed away before the rules were changed, whenever this was.
If there is any concern or you have any questions please do contact us.