"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: "
“… once the inspector comes to the conclusion that, on the facts which he has discovered, Mr Jonas has additional income beyond which he has so far declared to the Inspector, then the usual presumption on continuity will apply. The situation will be presumed to go on until there is some change in the situation, the onus of proof of which is clearly on the taxpayer.”
“The test to be applied, in my view, is to consider what a reasonable taxpayer, exercising reasonable diligence in the completion and submission of the return, would have done.” 36. The decision of Tax and Chancery Chamber of the Upper Tribunal in Colin Moore v HMRC [2011] UKUTT 239 (TCC) confirmed that the application of this test is a question of fact for the Tribunal. It is accepted that it is for HMRC to establish that Mr Brown was negligent in order for the penalties to be upheld. 37. The amount of the penalty is determined under s 100 TMA by an officer of HMRC setting it “ at such amount as, in his opinion, is correct or appropriate .”