Many common-law practitioners should be familiar with the competing theories on how to conceptualise the law of unjust enrichment.
Is the law still confined to an 'unjust factors' approach as recently reiterated by Lord Sumption in Test Claimants in the FII Group Litigation v HMRC  2 AC 337  (UKSC).
It is necessary, as the law presently stands, to bring the facts within one of the categories of case in which the law recognises that the recipient's retention of the money would be unjust.
Or should the late Peter Birks approach of an 'absence of basis' being the foundation of an unjust enrichment claim be given more merit? (See his 2005 book, Unjust Enrichment.) The approach seems to be rather successful in other jurisdictions such as the German courts.
META: I do not feel this is a question of opinion per se. This is serious academic discussion on a developing area of law, which imho should be the primary focus of discussion on Law.SE, and not questions on substantive answers/legal solutions to specific problems.