Lord Goff, dealing with the question of declaratory theory, said

The declaratory theory of judicial decisions

‘…we all know that in reality, in the common law as in equity, the law is the subject of development by the judges – normally, of course, by appellate judges. We describe as leading cases the decisions which mark the principal stages in this development, and we have no difficulty in identifying the judges who are primarily responsible. It is universally recognised that judicial development of the common law is inevitable. If it had never taken place, the common law would be the same now as it was in the reign of King Henry II; it is because of it that the common law is a living system of law, reacting to new events and new ideas, and so capable of providing the citizens of this country with a system of practical justice relevant to the times in which they live. The recognition that this is what actually happens requires, however, that we should look at the declaratory theory of judicial decision with open eyes and reinterpret it in the light of the way in which all judges, common law and equity, actually decide cases today.

When a judge decides a case which comes before him, he does so on the basis of what he understands the law to be. This he discovers from the applicable statutes, if any, and from precedents drawn from reports of previous judicial decisions. Nowadays, he derives much assistance from academic writings in interpreting statutes and, more especially, the effect of reported cases; and he has regard, where appropriate, to decisions of judges in other jurisdictions. In the course of deciding the case before him he may, on occasion, develop the common law in the perceived interests of justice, though as a general rule he does this “only interstitially,”… This means not only that he must act within the confines of the doctrine of precedent, but that the change so made must be seen as a development, usually a very modest development, of existing principle and so can take its place as a congruent part of the common law as a whole. In this process, what Maitland has called the “seamless web,” and I myself…have called the “mosaic,” of the common law, is kept in a constant state of adaptation and repair, the doctrine of precedent, the “cement of legal principle,” providing the necessary stability…

Occasionally, a judicial development of the law will be of a more radical nature, constituting a departure, even a major departure, from what has previously been considered to be established principle, and leading to a realignment of subsidiary principles within that branch of the law…

Bearing these matters in mind, the law which the judge then states to be applicable to the case before him is the law which, as so developed, is perceived by him as applying not only to the case before him, but to all other comparable cases, as a congruent part of the body of the law. Moreover when he states the applicable principles of law, the judge is declaring these to constitute the law relevant to his decision. Subject to consideration by appellate tribunals, and (within limits) by judges of equal jurisdiction, what he states to be the law will, generally speaking, be applicable not only to the case before him but, as part of the common law, to other comparable cases which come before the courts, whenever the events which are the subject of those cases in fact occurred.

It is in this context that we have to reinterpret the declaratory theory of judicial decision. We can see that, in fact, it does not presume the existence of an ideal system of the common law, which the judges from time to time reveal in their decisions. The historical theory of judicial decision, though it may in the past have served its purpose, was indeed a fiction. But it does mean that, when the judges state what the law is, their decisions do, in the sense I have described, have a retrospective effect. That is, I believe, inevitable. It is inevitable in relation to the particular case before the court, … it is also inevitable in relation to other cases in which the law as so stated will in future fall to be applied. I must confess that I cannot imagine how a common law system, or indeed any legal system, can operate otherwise if the law is be applied equally to all and yet be capable of organic change… I wish to add that I do not regard the declaratory theory of judicial decision, as I have described it, as an aberration of the common law…

[Edited for ease of reading.] You can find this report on Westlaw.