I apologise for the delay in answering your question. For some reason, the question was re-categorised and I could not access it.
You will find it hard to have the value changed, especially if there is no IHT liability in any event as you will read here
. Certainly, if it is correct, there will be no reason for HMRC to agree to a change. Those notes, however, really refer to a case where the property has been sold at less than probate value.
In your case, it is certainly worth to have the value amended if it was wrong as HMRC even provide a form to make an amendment to an IHT400 which you can find here
You mention both husband and wife in your last post. If by "and his capital gains tax allowance may be allowable" you are referring to the transfer of an unused Inheritance Tax allowance, you can read about that here
. The time limit for making such a claim is two years from the end of the month in which the second deceased died as you will read here
As long as the property was sold within the deceased estate within the period mentioned under the heading "Executors and personal representatives" here
, the estate is entitled to the annual CGT exemption for the tax year of death and each of the following two tax years depending when the disposal took place. The deceased estate is completely separate from the deceased person and all their assets cease to be theirs on their death and their tax allowances only apply against their income up to the date of death. The rate of CGT for a deceased estate is 28%.
I hope this helps. Let me know if you have any further questions.