“As to this the very use of the word 'judgment' makes it clear that the commissioners are required to exercise their powers in such a way that they make a value judgment on the material which is before them. Clearly they must perform that function honestly and bona fide. It would be a misuse of that power if the commissioners were to decide on a figure which they knew was, or thought was, in excess of the amount which could possibly be payable, and then leave it to the taxpayer to seek, on appeal, to reduce that assessment. Secondly, clearly there must be some material before the commissioners on which they can base their judgment. If there is no material at all it would be impossible to form a judgment as to what tax is due. Thirdly, it should be recognised, particularly bearing in mind the primary obligation, to which I have made reference, of the taxpayer to make a return himself, that the commissioners should not be required to do the work of the taxpayer in order to form a conclusion as to the amount of tax which, to the best of their judgment, is due. In the very nature of things frequently the relevant information will be readily available to the taxpayer, but it will be very difficult for the commissioners to obtain that information without carrying out exhaustive investigations. In my view, the use of the words 'best of their judgment' does not envisage the burden being placed on the commissioners of carrying out exhaustive investigations. What the words 'best of their judgment' envisage, in my view, is that the commissioners will fairly consider all material placed before them and, on that material, come to a decision which is reasonable and not arbitrary as to the amount of tax which is due. As long as there is some material on which the commissioners can reasonably act then they are not required to carry out investigations which may or may not result in further material being placed before them.”
“I have referred to the judgment in some detail, because there are dangers in taking Woolf J's analysis of the concept of 'best judgment' out of context. The passages I have italicised show that the tribunal should not treat an assessment as invalid merely because it disagrees as to how the judgment should have been exercised. A much stronger finding is required; for example, that the assessment has been reached 'dishonestly or vindictively or capriciously'; or is a 'spurious estimate or guess in which all elements of judgment are missing'; or is 'wholly unreasonable'. In substance those tests are indistinguishable from the familiar Wednesbury principles (see Associated Provincial Picture Houses Ltd v Wednesbury Corp[1948] 1 KB 223 ). Short of such a finding, there is no justification for setting aside the assessment.”
“…properly understood, the respondents’ task under s 73(1) of the 1994 Act is not…of a strictly mathematical or statistical nature at all; no doubt, any calculations forming part of the assessment have to be arithmetically accurate, but the power given to the respondents under statute is to make an estimate or an assessment to the best of their judgment on such information as is available to them. This necessarily allows the respondents a substantial margin of error. They are entitled to make what one might describe as an educated guess. They are not required to carry out exhaustive investigations.”
“In my view, the Tribunal, faced with a “best of their judgment” challenge, should not automatically treat it as an appeal against the assessment as such, rather than against the amount. Even if the process of assessment is found defective in some respect applying the Rahman (2) test, the question remains whether the defect is so serious or fundamental that justice requires the whole assessment to be set aside, or whether justice can be done simply by correcting the amount to what the Tribunal finds to be a fair figure on the evidence before it. In the latter case, the Tribunal is not required to treat the assessment as a nullity, but should amend it accordingly.”