GOV.UK
Succession: Investigating boxes 24 to 28 of the IHT400: General approach
You should generally assume (unless you have reason to suspect otherwise) that the information provided by the taxpayer or agent on the IHT400 is correct.
If questions 24 to 28 have not been completed correctly and in full then you should normally ask the taxpayer or agent to answer the outstanding questions - unless you have information in the Will or elsewhere on the IHT400 to suggest that further enquiries will not result in additional tax. If the taxpayer or agent has not followed the instruction (IHTM12011) to attach a copy of the Will to the IHT400 you should write out or phone for a copy Will.
Caseworkers in Service should only ask for a copy Will if they are asked to do so by the risk assessors.
If you do need to ask for information that should have been supplied you should point out to the taxpayer or agent that they have not provided the details requested in the form IHT400.
If a copy of an instrument of variation (IHTM35011) or disclaimer (IHTM35161) is attached to the IHT400 then you should consider the Will in conjunction with the relevant instructions (IHTM35000).