R v East Sussex County Council, ex p Reprotech (Pebsham) Ltd (HL)  UKHL 8; 2002] 4 All ER 58.
....I think that it is unhelpful to introduce private law concepts of estoppel into planning law. As Lord Scarman pointed out in Newbury DC v Secretary of State for the Environment, Newbury DC v International Synthetic Rubber Co Ltd  1 All ER 731 at 752,  AC 578 at 616, estoppels bind individuals on the ground that it would be unconscionable for them to deny what they have represented or agreed. But these concepts of private law should not be extended into 'the public law of planning control, which binds everyone'. (See also Dyson J in R v Leicester City Council, ex p Powergen UK Ltd  4 PLR 91 at 100.)
 There is of course an analogy between a private law estoppel and the public law concept of a legitimate expectation created by a public authority, the denial of which may amount to an abuse of power (see R v North and East Devon Health Authority, ex p Coughlan (Secretary of State for Health intervening)  3 All ER 850,  QB 213). But it is no more than an analogy because remedies against public authorities also have to take into account the interests of the general public which the authority exists to promote. Public law can also take into account the hierarchy of individual rights which exist under the Human Rights Act 1998, so that, for example, the individual's right to a home is accorded a high degree of protection (see Coughlan's case  3 All ER 850 at 883-884,  QB 213 at 254-255), while ordinary property rights are in general far more limited by considerations of public interest (see R (on the application of Alconbury Developments Ltd) v Secretary of State for the Environment, Transport and the Regions  UKHL 23,  2 All ER 929,  2 WLR 1389).
 It is true that in early cases such as Wells' case and Lever (Finance) Ltd v Westminster Corp  3 All ER 496,  1 QB 222, Lord Denning MR used the language of estoppel in relation to planning law. At that time the public law concepts of abuse of power and legitimate expectation were very undeveloped and no doubt the analogy of estoppel seemed useful. In the Western Fish case the Court of Appeal tried its best to reconcile these invocations of estoppel with the general principle that a public authority cannot be estopped from exercising a statutory discretion or performing a public duty. But the results did not give universal satisfaction (see the comments of Dyson J in Ex p Powergen  4 PLR 91 at 100-101). It seems to me that in this area, public law has already absorbed whatever is useful from the moral values which underlie the private law concept of estoppel and the time has come for it to stand upon its own two feet.