Go on, my son!

I've always had difficulty getting my head round this legal doctrine of joint enterprise, or common purpose as it is sometimes called, and I don't think the recent Supreme Court ruling has helped me. In English law, to be guilty of a crime, you must have done something which is against the law (the actus reus) and you must have done so in a state of mind which is against the law (the mens rea).  For murder, the actus reus is obviously killing someone, and the mens rea is doing so intending to kill someone (not necessarily the person you did kill) or at the very least intending to cause them really serious harm. OK - you're part of a gang which attacks someone and that someone dies.  You do not strike the fatal blow but you really wish this person dead and generally join in the attack and shout encouragement. So you probably have the necessary mens rea but where is the actus reus?  That was committed by someone else.  You may have a moral responsibility for the person's death, but legally responsible?  I've mentioned before that the Bentley/Craig case back in the early 50s was instrumental in me taking an interest in the law and what I saw as the failings of the system.  Not sure I'm any further forward now.