“It is important to start by stating that it is common ground that the jurisdiction of the tribunal is only supervisory. The appeal before the tribunal is not by way of a rehearing (see, for example Customs and Excise Comrs vJH Corbitt ( Numismatists ) Ltd[1980] STC 231 at 239,[1981] AC 22 at 60 per Lord Lane). This was accepted in the present case by the chairman himself. He put the matter clearly and, in my view, accurately in his decision in these terms: 'The jurisdiction of the tribunal in cases such as this where the Commissioners are exercising discretionary powers has been clearly established in previous cases. It is, for instance, clear that the tribunal cannot substitute its own discretion for that of the Commissioners for the tribunal has no discretion in these matters. If it is alleged that the Commissioners have reached a wrong decision then there can be a question of law but only of a limited character. The question would be whether their decision was unreasonable in the sense that no reasonable panel of Commissioners properly directing themselves could reasonably reach that decision. To enable the tribunal to interfere with the Commissioners' decision it would have to be shown that they took into account some irrelevant matter or had disregarded something to which they should have given weight.' In my judgment, in exercising its supervisory jurisdiction the tribunal must limit itself to considering facts and matters which existed at the time the challenged decision of the commissioners was taken. Facts and matters which arise after that time cannot in law vitiate an exercise of discretion which was reasonable and lawful at the time that it was effected.”
“the Commissioners will not arrive reasonably at a decision if they take into account irrelevant matters, or fail to take into account all relevant matters”