Mason's speech starts:

...

Each edition of Goff and Jones’ Law of Restitution commences:

"Restitution is the law relating to all claims, quasi-contractual or otherwise, which are founded upon the principle of unjust enrichment. Restitutionary claims are to be found in equity as well as at law."

On the other hand, as I understand it, the Birksian view is different. For instance, my copy of Birks, Unjust Enrichment 2nd edition avers:

... it was an error to regard unjust enrichment as coterminous with restitution. It was no more than a part of the law of restitution when that area of law was divided according to the different events from which restitutionary rights arose.