'The situation to which paragraph (a) is addressed arises where the effect of the primary legislation is that the authority has no alternative but to do what the legislation tells it to do. The language of the paragraph tells us that this may be the result of one provision taken by itself, or that it may be the result of two or more provisions taken together. Where more than one provision is involved, they may be part of one enactment or they may be found in several different enactments. The key to its application lies in the fact that the effect of this legislation, wherever it is found, is that a duty is imposed on the authority. If the legislation imposes a duty to act, the authority is obliged to act in the manner which the legislation lays down even if the legislation requires it to act in a way which is incompatible with a Convention right. The authority has no discretion to do otherwise. As it is a duty which has been imposed on the authority by or as a result of primary legislation, Parliamentary sovereignty prevails over the Convention right. The defence is provided to prevent the legislation from being rendered unenforceable. The situation to which paragraph (b) is addressed on the other hand arises where the authority has a discretion, which it has the power to exercise or not to exercise as it chooses, to give effect to or enforce provisions of or made under primary legislation which cannot be read or given effect to in a way which is compatible with the Convention rights. The source of that discretion may be in a single statutory provision which confers a power on the authority which it may or may not choose to exercise.'