#illegality #judicial-review #public

The House of Lords' decision in R v Lord President of the Privy Council, ex parte Page [1993] 1 All ER 97 confirmed that all errors of law are reviewable. However, there are some qualifications to this general principle, as follows.
(a) Where the error of law is not decisive to the decision. By 'decisive', it is meant that, but for the error concerned, the decision would have been different.
(b) Where the decision-maker is interpreting some special system of rules, courts are often unwilling to intervene, for example, with statutes of a university. (This affected the actual decision in Page). This can also be extended (controversially) to a decision made by an inferior court, where it is deemed that Parliament had expressly provided that the decision of a judge at first instance was to be final, as in Re Racal Communications Ltd [1981] AC 374.
(c) Where the power granted is so imprecise that it is capable of being interpreted in a wide range of different ways, the courts will not always quash a decision just because they would have come to a different view on what the law requires from that of the decision-maker. In R v Monopolies Commission ex p South Yorkshire Transport Ltd [1993] 1 All ER 289, the House of Lords had to consider how to interpret how the phrase 'a substantial part of the United Kingdom' should be interpreted. Lord Mustill observed that [...]. His view was that the word was so broad and imprecise that different decision-makers might reach differing conclusions when applying them to the facts of a given case. In this situation, he held that the court should not intervene unless the decision-maker's conclusion was irrational.

'substantial' accommodates a wide range of meanings and warned of the dangers of attaching a spurious degree of precision on an inherently imprecise word