How does 'judgement' differ from 'discretion'?

Question

Source: p 52, The Rule of Law (2010) by Thomas Bingham

In statutes the word ‘may’ (as opposed to ‘shall’ or ‘must’) is ordinarily understood to confer a discretion: the judge (or the minister, or whoever) may do whatever it is, but is not bound to do so. Thus, while a judge in a criminal trial ordinarily has no discretion to refuse to allow evidence to be given which is admissible under the rules governing the admissibility of evidence, section 78(1) of the Police and Criminal Evidence Act 1984 creates a discretionary exception. The section says:

_{In any proceedings the court may refuse to allow evidence on which the prosecution proposes to rely to be given if it appears to the court that, having regard to all the circumstances, including the circumstances in which the evidence was obtained, the admission of the evidence would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it.}

The court ‘may’: therefore it has a discretion. But it is a discretion which may only be exercised if – a big if – it appears to the court that having regard to the matters mentioned the admission of the evidence would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it (as might be so, for example, if it were shown that a witness had been tricked or bribed into giving a statement). Whether or not it so appears to the court calls for an exercise of judgment, but not an exercise of discretion. It either does or does not appear to the court.

Will you please aid me to understand the bold: Lord Bingham's bifurcation of 'judgement' and 'discretion'? I do not understand how the above example pertains to judgement, and not discretion.

Answer 1

First of all the court must apply judgement to decide:

if it appears to the court that, having regard to all the circumstances, including the circumstances in which the evidence was obtained, the admission of the evidence would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it.

If in their judgement it does then they have discretion as to whether or not to allow it. If in their judgement it does not then they have no discretion and must allow it.

An appeals court can examine the use of judgement but not the use of discretion. However, it is difficult to imagine a circumstance where any judge, having judged that the evidence is unfair, would exercise their discretion and allow it anyway.

A lot of this discretion power has to do with the separation of powers: the legislature can't tell judges what to do in many circumstances. This looks like the legislature would like to tell the judge they have to admit evidence that is unfair, but can't because that would be unconstitutional.