General assessment procedures: Disclosure of calculations
The trader has a right to know how an assessment has been calculated and HMRC would be quite properly criticised if the information was not provided.
Where possible you should give an explanation of any assessment to the trader during the visit and at the same time explain the basis of calculations and principles involved. If an explanation was not given, for example because of the trader’s absence, this should be noted in the visit report.
Your pre-assessment letter should in any event supply full details of the basis of the assessment.
This will enable the trader to decide whether to accept or dispute the proposed assessment and gives the opportunity to make further representations before the assessment is made.
Should the trader still be unclear, you may need to give further assistance in response to any reasonable request from the trader or the trader’s representative for additional details or explanations.