there are two main ways in which, in a case of this kind, evidence of an offence allegedly committed on one occasion may be relevant to an allegation that the defendant committed an offence on another occasion, either against the same or against a different complainant. One way in which such evidence may be relevant is if it goes to establish a propensity to commit a particular kind of offence. The basic reasoning is that, if he has done similar things on other occasions, it is more likely that he did it on this occasion... The second main way in which evidence relating to one alleged offence may be relevant to the issue of whether the defendant committed another alleged offence is simply by reducing the likelihood of there being an innocent explanation for the allegations. So, for example, in a case such as the present one, where two individuals each make allegations that they have been sexually assaulted by the same person, provided there is no reason to think that their allegations are linked for some other reason – for example, because they had got together to concoct false stories, the evidence of each complainant may strengthen the case relating to the other.
Both of these categories of case involve the use of evidence which is evidence of the defendant's bad character and the admission of such evidence is therefore governed by the relevant provisions of the Criminal Justice Act 2003. Furthermore, under Part 21 of the Criminal Procedure Rules, where a party wants to introduce evidence of bad character, there is a procedure which must be followed which involves the service of a notice and, if objection is taken to it, an application to the court to rule on the matter.
Looking at the matter more broadly, the general tendency of the criminal law over time has been towards a gradual relaxation of rules of evidence and an increasing willingness to trust to the good sense and rationality of juries to judge for themselves whether particular evidence is relevant to an issue they have to decide and if so in what way. But we have not yet reached the point where evidence of a defendant's bad character can be left as a free for all. The particular ways in which evidence that a person has committed one offence may or may not be relevant in deciding whether that person is guilty of another offence are not always immediately obvious even to legal professionals and have had to be worked out by the courts in a number of cases. Lay jurors are entitled to assistance on these questions and cannot be expected to work out the approach which the courts regard as proper for themselves. It therefore seems to us to be essential that, in a case of this kind, the jury should be given clear directions on whether, and if so how, evidence relating to one count may be taken into account in deciding guilt on another count.