I got this from an article in the Telegraph from a few days ago. Assuming it's accurate, you could pass on a lot more to your children if you get married.
Currently, IHT is charged at 40pc on estates that are worth more than £325,000. However, if you are married or in a civil partnership, all assets can be passed to a surviving spouse without any inheritance tax being applied whatsoever.
What's more, when the second spouse dies you can utilise both partners' allowance when passing assets on to the next generation. This means married couples can leave £650,000 to their children before IHT is applied.
If I understand this right, what it means is that when the first spouse dies, if everything goes to the surviving spouse (and it does have to be a spouse or CP) there is no IHT to pay at all, so the nil rate band of £325k doesn't get used.
When the second spouse dies and the estate passes to their heirs (whoever they are), because the first spouse didn't use their £325k, that gets applied to the second spouse's estate along with the second spouse's own nil rate band, and that knocks £650k in total out of IHT altogether.
Hope this helps.