This decision has little, if anything, to do with the Human Rights Act or the European Convention on Human Rights. It is essentially a matter of criminal law and fundamental principles of natural justice which we had firmly embedded in our system long before the HRA came into being. It isn't in anyone's interests for the innocent to be convicted, and in particular it isn't in the interests of the victim.
I think it's a real pity that in this sort of situation journalists go rushing off to the victims and their relatives and start winding them up by implying that the murderers are about to go free. When the recent interpretation of the law is applied, the majority of those currently in prison convicted of joint enterprise will stay there, because the evidence against them will amply meet the requirements of the revised test. If victims and their relatives are upset by this, they have the newspapers, particularly the right wing press, to blame rather than the courts.