IHTM10081 - Premium Bonds (box 54): investigating values
The total value of all Premium Bonds owned by the deceased should be included in box 54 and at box 3 on form IHT406 This should also include any unclaimed or uncashed prizes (IHTM10082).
In practice, you should normally accept the value given. You should only raise enquiries if you have information from elsewhere to suggest that the value of the bonds has been understated or prizes have not been included.