VATAC2900 | Use of estimation for completing VAT returns: review of existing approvals
From HM Revenue & Customs · VAT Accounting Manual
In reviewing an agreed estimation method, you should satisfy yourself that:
there is continuing need for a concession (against a background of ever more sophisticated computer systems and greater business efficiency);
the conditions of approval are being observed; and
the method of estimation does not result in regular understatements of output tax or overstatements of input tax.
Where necessary, be prepared to suggest and discuss with businesses suitable changes to their method of calculation, to produce a fair and equitable result.