The current Franked Investment Income (“FII”) group litigation raises a number of interesting and hitherto untested issues regarding the availability of and limits to unjust enrichment claims brought against the Revenue. In particular, the litigation raises the novel question whether the defence of change of position applies to claims for restitution of tax paid to the Revenue under legislation which contravenes European Union law. The financial consequences of the answer are momentous, likely running into billions of pounds. The legal ramifications are also highly significant. The decision of Henderson J usefully highlights some unsettled questions on the nature and scope of the change of position defence, particularly in its application to claims brought against public authorities, even if it ultimately fails to answer them satisfactorily.