In 2011 HMRC announced rules to encourage people to leave part of their estate on death to charity.  

This reduces the rate of inheritance tax (IHT) payable on death if the deceased leaves 10% of the chargeable estate to charity. 
Since gifts to charity are already exempt from IHT, this enables the remainder of the estate to pay tax at just 36%, rather than 40%.
This means gifts made while still alive, in the twilight years, should be stored up for payment from the net estate, if they are to come to an amount of more than 10% of the net chargeable estate. The effect can be dramatic!
But the key is to work from the net estate. So this is 10% of the net after the nil rate band.
For those interested in this, (and advisers active in will writing will understand this) that a clause should be inserted so that a specific legacy to charity will always meet the 10% test.  Otherwise if all beneficiaries and executors agree, a deed of variation can be drawn up to vary the will after death to allow for this.
The Society of Trust and Estate Practitioners have drafted a model clause to satisfy this test available here.