“If a provision requires the public authority to take a particular step which is, of its very nature, incompatible with Convention rights, then no process of interpretation can remove the obligation or change the nature of the step that has to be taken….. The only cure is to change the provision and that is a matter for Parliament and not for the courts … [H]owever powerful the obligation in s.3(1) may be, it does not allow the courts to change the substance of a provision completely, to change a provision where Parliament says that x is to happen into one saying that x is not to happen.”