IHTM10092 - National Savings investments: investigating values
Taxpayers can find out the value of all National Savings investments by writing to National Savings and Investments. Taxpayers may not always provide full details of National Savings investments on form IHT406. There is no need to call for this information if the value given is relatively small and there is no clear evidence of any significant undervaluation.
However, amounts shown as estimates should usually be questioned and you may wish to ask the taxpayers to provide evidence of the value in a letter from National Savings.