“If an officer of Revenue and Customs discovers as regards an accounting period of the company that: (a) an amount which ought to have been assessed to tax has not been assessed, or (b) an assessment to tax is or has become insufficient, or (c) relief has been given which is or has become excessive. The officer may make an assessment (a “discovery assessment”) in the amount or further amount which ought in their opinion to be charged in order to make good to the Crown the loss of tax.”
“The element of guesswork and the almost unavoidable inaccuracy in a properly made best of judgment assessment, as the cases have established, do not serve to displace the validity of the assessments, which are prima facie right and remain right until the taxpayer shows that they are wrong and also shows positively what corrections should be made in order to make the assessments right or more nearly right." (Bi−Flex Caribbean Ltd v Board of Inland Revenue (1990) 63 TC 515, 522−3 PC, per Lord Lowry)”