the CPO, then it becomes very difficult to challenge. Once the CPO is confirmed, the purchasing authority must serve a Notice to Treat within three years, and a Notice of Entry within a further three years. It may take possession of the land not less than 14 days after serving the Notice of Entry. The Notice to Treat requires the land's owner to respond, and is usually the trigger for the land's owner to submit a claim for its value. If no claim is submitted within 21 days of the Notice to Treat, the acquirer can refer the matter to the Lands Tribunal. If the land's owner cannot be traced and does not respond to a Notice to Treat affixed to the land, then the purchasing authority must pay the compensation figure to the Court. Crichel Down rules The Crichel Down principles oblige central and local government, when, having acquired an estate compulsorily, they find they no longer need it, to offer it in the first instance to the person from whom they acquired it at its market value. However, this only applies where the land has not materially changed in character, and does not withstand the principle that councils may not dispose of land "for a consideration less than the best that can be obtained" under the Local Government Act 1972, section 123. This means that where it is difficult to value land for some reason, the land may need to be sold by tender or auction. Human rights Because of property's social importance, either for personal consumption and use or for mass production, compulsory purchase laws have met with human rights challenges. One concern is that since the 1980s privatisations, many compulsory purchase powers can be used for the benefit of private corporations whose incentives may diverge from the public interest. For example, the Water Resources Act 1991 continues to allow government bodies to order compulsory purchases of people's property, although profits go to the private shareholders of UK water companies. In R (Sainsbury's Supermarkets Ltd) v Wolverhampton CC the Supreme Court held that Wolverhampton City Council acted for an improper purpose when it took into account a promise by Tesco to redevelop another site, in determining whether to make a compulsory purchase order over a site possessed by Sainsbury's. Lord Walker stressed that "powers of compulsory acquisition, especially in a 'private to private' acquisition, amounts to a