I disagree - you seem to want your cake and eat it. I don't know whose valuation you say it was below but public bodies have rules under CPO about using independent professional valuers to determine the CPO price.
If you want the land back I think you are going to have to first agree that the terms of the original CPO should have gone through. There is (and always was) a process for agreeing the price, including appeals, involving valuers.
Second point - there is no obligation on the public body to use the land within a certain time.
Third point - there is an obligation to first offer the land back to the original owner if it is decided the land is no longer needed provided the land remains broadly as it was when initially CPOed. Guidelines for Government Departments follow an obscure piece of caselaw known as Critchel Down Rules. This dates back to a case involving land acquired during WW2 by the Government and subsequently sold on many years later; the original landowner took the case to court and won.
Here is the Government Circular on CPO.
http://www.communities.gov.uk/documents/planni ngandbuilding/pdf/142928.pdf
It is a massive 124 pages, applies in England and Wales, and the bit that you need to look at starts around page 108 - where it talks about Critchel Down Rules and selling back.