“Having considered these authorities my conclusions are as follows: (1) In a case such as the present, involving a challenge to a decision of a public body in relation to a contract, it is necessary to consider: (a) by reference to the contract in question, to the relevant statutory power, to the statutory framework (if relevant), and to all other relevant matters, whether or not, and if so to what extent, the defendant is exercising a public function in making the decision complained of; (b) whether, and if so to what extent, the grounds of challenge involve genuine and substantial public law challenges to the decision complained of, or whether, and if so to what extent, they are in reality private law challenges to decisions made under and by reference to the terms of the relevant contract. (2) In a case involving a challenge to a decision of a public body acting under a statutory power but in relation to a contract and in the absence of a substantial public function element, a claimant will nonetheless normally be entitled to raise genuine and substantial challenges based on fraud, corruption, bad faith, and improper motive (in the sense identified by De Smith of the knowing pursuit of an improper purpose). (3) The extent to which a claimant will be entitled to raise genuine and substantial public law challenges beyond those limited classes will depend on a careful analysis of all of the relevant circumstances so as to see whether or not there is a relevant and sufficient nexus between the decision in relation to the contract which is challenged and the grounds complained of.”