'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.' (See Bi-Flex Caribbean Ltd v The Board of Inland Revenue (1990) 63 TC 515 at 522-523 per Lord Lowry.) That was confirmed by this court, after a detailed review of the authorities, in Customs and Excise Comrs v Pegasus Birds Ltd[2004] EWCA Civ 1015 ,[2004] STC 1509 . We also cautioned (see[2004] STC 1509 at [38]) 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: '