A few words should be said about the application of restitutionary rules to public authorities.1 The law of restitution deals with situations in which the defendant has acquired some benefit at the expense of the plaintiff and it can be said that the benefit has been unjustly obtained. Suppose a public authority charges a citizen for a service which it is under a duty to provide free of charge, or for some lesser amount than it actually charges. If the authority threatened to withhold the service if the charge was not paid then the law of restitution would consider that the charge had been unjustly extracted and would allow the citizen to recover it.2 Similarly, if an authority made an unlawful tax demand and extracted the payment by applying unlawful pressure to the taxpayer, any payment might be recoverable.
But what if the authority makes no threat or applies no pressure? Suppose that it believes, wrongly, that it is, as a matter of law, entitled to make the charge3 and the citizen does not realize that it need not be paid. Or suppose that an authority makes a wrongful tax demand thinking that, as a matter of law, the tax is due, and that the taxpayer pays it on the same basis. As a matter of public law, the tax or charge will be illegal,4 and in 1992 the House of Lords decided t+hat illegal taxes and charges are recoverable in the law of restitution by virtue of the constitutional principle of 'no taxation without Parliamentary approval'.5____________________