The existence and content of Constitutional Conventions is a question of fact, not positive law. They are rules that exist outside of the positive legal order. They provide a set of rules for how legal powers may be exercised. A change to those legal powers may, and usually will, lead to either the change or abolition of the Conventions which relate to those powers.
When a judge needs to decide the outcome of a case, she must commonly rule not only on questions of law but of fact also. This means that rules from outside of the legal order may be relevant to the outcome of a dispute. Some simple examples of this may be given
-a claimant is injured in a car accident in Switzerland, and brings a claim against the defendant driver in England. If raised, the law to be applied to their dispute will be Swiss law. Foreign law is, before the English courts, as a matter of fact.
-a contract may provide that the question of how it is to be performed is to be governed by the principles of Sharia law. If a dispute under the agreement came before an English court, it may be necessary to examine what Sharia law requires, as a matter of fact, in order to determine what the parties have agreed.
Similarly, in the infamous Prince Charles Black Spider Memos case, the issue of what the Constitutional Convention of the education of the heir to the throne should be instructed on the business of government was relevant to what the ‘public interest’ required for purposes of disclosure under the Freedom of Information Act 2000. However, this was a matter to be determined by the trier of fact, the existence and meaning of which could not be directly addressed on appeal on the issue of law. See the (long and tedious) decision of the Supreme Court
One judge’s ruling on an issue of fact is not binding on subsequent judges in different disputes between different parties. A court cannot therefore authoritatively rule for the purpose of subsequent disputes on the existence and meaning of a Constitutional Convention.
Are such Conventions part of the Constitution then?
The answer to that question is, as with all good ones, it depends what you mean. What does the Constitution constitute?
If by the Constitution we only mean a body of positive law, then no, as Conventions form no part of the positive law. If by the Constitution we mean the body of rules, both legal and non-legal, then yes Conventions form a central part of the UK Constitution.