"(1) In criminal proceedings evidence of the bad character of a person other than the defendant is admissible if and only if — … (b) it has substantial probative value in relation to a matter which — (i) is a matter in issue in the proceedings, and (ii) is of substantial importance in the context of the case as a whole …"
"(1) Subject to subsection (2), a reference in this Chapter to the relevance or probative value of evidence is a reference to its relevance or probative value on the assumption that it is true. (2) In assessing the relevance or probative value of an item of evidence for any purpose of this Chapter, a court need not assume that the evidence is true if it appears, on the basis of any material before the court (including any evidence it decides to hear on the matter), that no court or jury could reasonably find it to be true."
"36. … a fact specific judgment directed to the statutory conditions in section 100(1)(b) and section 100(3) must be made whether to admit evidence of bad character (within the ambit of section 98(a) of the [2003] Act). Where it applies, the assumption in section 109 is not determinative of the admissibility question. Rather it provides the context in which the admissibility decision falls to be made. In short, the pre-conditions to admissibility under section 100(1) are not automatically established, and, notwithstanding the evidential assumptions provided by section 109 at the admissibility stage, the bare fact of an allegation (even if assumed to be true) is not necessarily conclusive of the question whether it constitutes substantial probative evidence or evidence of substantial importance in the context of the case as a whole. If it were otherwise, the court would be obliged to admit evidence of an allegation of a serious crime allegedly committed by the witness, even if it had been fully investigated by the police, but, because the investigation revealed serious doubts about the complainant's veracity, on the basis that the complainant continued to insist that the allegation was true. Accordingly, … we do not see how the necessary judgment whether the pre-conditions to admissibility under section 100 can be satisfied without a careful examination of all the material which bears on the question. 37. A trial concerned with whether it is proved that the defendant has committed crime 'A' is liable to be derailed if the jury is required to decide whether a witness has committed the distinct, separate crimes, 'B' and 'C'. As we have explained, the evidential assumption in section 109 does not bind the jury, and the investigation of this evidence at trial may be liable to distract attention from the crucial issue which is whether the case against the defendant has been proved. If, in the context under discussion, the judge correctly directs the jury that they must not consider the alleged bad character evidence unless they are sure that it is true, two trials would be simultaneously in progress before the same jury. First, the trial of the defendant for the crime alleged against him by the prosecution, and second, the crime or misconduct alleged against the witness. 38. In our judgment these are relevant considerations bearing on the assessment of the probative value of the evidence sought to be adduced and its importance in the overall context of the case. When it is assessing the probative value of the evidence in accordance with section 100(1)(b) and section 100(3), and consistently with section 100(2), among the factors relevant to the admissibility judgment, the court should reflect whether the admission of the evidence relating to the bad character of the witness might make it difficult for the jury to understand the remainder of the evidence, and whether its understanding of the case as a whole might be diminished. In such cases the conclusion may be that the evidence is not of substantial probative value in establishing the propensity in or lack of credit worthiness of the witness, or that the evidence is not of substantial importance in the context of the case as a whole, or both. If so, the pre-conditions to admissibility will not be established."