“The element of guess-work 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). That was confirmed by this court, after a detailed review of the authorities, in Customs and Excise Commissioners v Pegasus Birds Ltd[2004] STC 1509 ;[2004] EWCA Civ 1015 . We also cautioned against allowing such an appeal routinely to become an investigation of the bona fides or rationality of the "best of judgment" assessment made by Customs: “The tribunal should remember that its primary task is to find the correct amount of tax, so far as possible on the material properly available to it, the burden resting on the taxpayer. In all but very exceptional cases, that should be the focus of the hearing, and the Tribunal should not allow it to be diverted into an attack on the Commissioners' exercise of judgment at the time of the assessment.” (para 38(i)) It should be noted that this burden of proof does not change merely because allegations of fraud may be involved (see e.g. Brady v Group Lotus Car Companies plc[1987] STC 635 , 642 per Mustill LJ).”