R v Rahman and others [2008] UKHL 45, [2008] All ER (D) 47 (Jul)
If B realises (without agreeing to such conduct being used) that A may kill or intentionally inflict serious injury, but nevertheless continues to participate with A in the venture, that will amount to a sufficient mental element for B to be guilty of murder if A, with the requisite intent, kills in the course of the venture unless: (i) A suddenly produces and uses a weapon of which B knows nothing and which is more lethal than any weapon which B contemplates that A or any other participant may be carrying; and (ii) for that reason A’s act is to be regarded as fundamentally different from anything foreseen by B (per Lord Brown at 68).
