After deducting all gifts to ones spouse, and to charities, there is a nil rate band of �275,000 on the net estate, after which the rate leaps to 40%. However certain lifetime gifts made within the preceding 7 years can be added back in to tthe value of the estate to calcualte the IHT liabilityy.
The 40 per cent is only on the amount above the inheritance tax limit, not on the whole estate. For example, if the limit which Didwot quotes is correct ( I haven't checked myself, but I know it's somewhere around there) if the total estate was �300,000 you would pay 40 per cent tax on �25,000.