The answer to your question appears to be within the notes associated with the IHT205 form that you filled in when establishing whether there was any IHT to pay.
It says that it is permissible to enter your best estimates on the value of an asset or assets.
Then further down the form, in the calculation summary it states: -
"If you find something has been left out, or if any of the figures you have given in this form change later on, you only need to tell us if, taking all the omissions and changes into account, the value at box K is more than the Inheritance Tax nil rate band (or two times the nil rate band where form IHT217 has been submitted). You must then fill in form IHT400 and send it to us. You will also need to pay any tax that is due."
Since you've been through the process, I'm sure you know what that means - no need unless it takes the estate over the threshold for paying IHT.
BM