LJ Sales suggests that "a referendum on any topic can only be advisory for the lawmakers in Parliament unless very clear language to the contrary is used in the referendum legislation in question" (p 106).
But this is a suggestion, not a citation of a current Statute. It is also a bad suggestion.
It is bad practice to use in legislative or any other government documents words contrary to their natural intuitive linguistic meaning, even if this unnatural meaning is explained in some other document. This leads to confusion and misunderstandings, which at the very least make the work of the people working with such documents more difficult, and can lead conflicts, unnecessary litigation and appeals.
Legal documents should use words in their intuitive natural meaning, and if there is any special use of such word, it should be made clear within the document, and preferably next to the word.
The natural intuitive linguistic meaning of the word "referendum" implies that the decision will resolve the issue and the government will be obliged to implement it, rather than that it would be used to provide advisory information to the government which it will be free to follow or not.
This is clear from the fact that the voting public, and the Government saw Referendum 2016 as binding. And, it was only when the "Remain Camp" learnt that the decision was "leave", that the view started being promoted that the Referendum was "advisory". Without qualifying the word "referendum" by an adjective "advisory", the word "referendum" had the clear natural meaning of "binding referendum".
Yes, it is possible to enact a statute that would change the unqualified meaning of the word "referendum" from binding to advisory, as LJ Sales suggests, and to require that a binding referendum should always be qualified by the adjective "binding", and failure to do so would make it advisory.
But such unnatural change of linguistic meaning of words is just as absurd and unnecessary as to enact a statute that the meaning of the word "marriage" should be "same sex civil partnership" unless qualified by the adjective "heterosexual". This would be clearly contrary to the commonly accepted meaning of that word.
But it is not enough to use words in their natural meaning in government documents.
To avoid confusion, as exemplified by the present controversies around Referendum Act 2016, every Act of Parliament must have (1) a clear statement of the reasons for its enactment, (2) a clear statement of the purpose which the Act is intended to achieve, (3) description of how and by whom the Act is to be implemented, and (4) description of the criteria and methods of determination of whether and to what degree the Act is achieving its intended purpose.
If Referendum Act 2016 had these sections, the controversies about it, and the court proceedings that followed would have been unnecessary.
The practice of drafting legal documents, the meaning of which cannot be clear, without seeking a court declaration, is an example of careless incompetent governance.