R v Blastland  AC 41 is a case concerning the admissibility of evidence, specifically the threshold of probative value.
Blastland was charged with buggery and murder of the 12 year old KF, B admitted to paying KF for consensual intercourse but claimed that KF left him unharmed, and that KF was murdered by M. This was backed up by the fact that M came back home to the woman he was living with in an agitated and disheveled manner claiming that a child has died today before the murder was reported. The defence counsel was not permitted to reveal these facts to the jury at trial.
Upon appeal to the House of Lords, the court upheld the decision by saying that it lacked "direct and immediate relevance to an issue which arises at trial", as such knowledge could have came able in a number of different ways, some incriminating whilst others completely innocent, juries would not have been in a position to judge how Mark's knowledge came about.
Lord Bridge's direct and immediate relevance was a variation of the concept of legal relevance, as he casually referred to "degree of relevance" hence suggesting the fact that his lordship is working within a conception of relevance with an inherent criterion of weight or quantum; hence legal relevance.
The defence clearly wanted the jury to examine on how M was able to get a hold of the knowledge, which will distract the juries from arriving at a verdict. But on the other hand it could also be argued that this ambiguity gives raise reasonable doubt thus giving the juries grounds for B's acquittal, rather than distracting them.
Blastland stands for the paradoxical proposition that obviously (according to common sense) relevant information are legally irrelevant. It would be better argued that although the information is relevant the trouble it generates outweighs its marginal probative value. This is difficult to consistently rationalise this with the protection of the innocent.
According to Andrew Choo (The notion of relevance and defence evidence Crim. L.R. 1993, Feb, 114-126) it is obvious that the evidence had probative value and that the minimum threshold of probative value posed by Lord Bridges exceeded that of the bare minimum, "direct and immediate relevance". This inevitably rises the question why is this higher standard employed in Blastland? Perhaps his lordship felt that such evidence would have led to much confusing collateral issues which could confuse the jury, but in any event his reasons were left unarticulated. Choo suggests this above bare minimum threshold of relevance is too much burden for the defence, had it been only for the prosecution then it would have been more justified because the LoE has a duty to protect the innocent from wrongful conviction and denying evidence adduced by the defence is not protecting them from wrongful conviction.
The evidence would have been admissible if it is able to actually suggest that Mark had obtained this knowledge as a perpetrator, rather than merely rise the possibility that he did. Also note the fact that "direct ad immediate relevance" is above the bare minimum Probative Value.