R. v. MMC, ex p. S. Yorks Transport [1993]

was whether area covering 3.2% of total UK population was ‘substantial part of
the UK’
Mustill held that term ‘substantial’ was open to wide range of possible
meanings and it was up to court to decide where on spectrum to place it.
criterion chosen might be so imprecise that different decision-makers, each
acting rationally, could reach different conclusions.
such a case court can only substitute its own opinion if decision is so
aberrant that it cannot be classed as rational.
present case, MMC’s conclusion was within permissible field of judgement.