“In my view, the tribunal faced with a best of their judgment challenge should not automatically treat it as an appeal against the assessment itself, rather than against the amount. Even if the process of assessment is found defective in some respect..., 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”