A friend asked me to look again at Evangelou vs McNicol, which I did, by looking at Evangelou vs Mcnicol Appeal Judgment 20160812
The clause that interests us all is,
24. In the present case, there is no challenge to the rationality of the eligibility criteria and the freeze date, and they are only said to be unauthorised on the true construction of the contract. It is, however, relevant to note that a discretion conferred on a party under a contract is subject to control which limits the discretion as a matter of necessary implication by concepts of honesty, good faith and genuineness, and need for absence of arbitrariness, capriciousness, perversity and irrationality: see Sochimer International Bank Ltd v Standard Bank London Ltd [2008] EWCA Civ 116, [2008] Bus LR 134 at [66] and Braganza v BP Shipping [2015] UKSC 17, [2015] 1 WLR 1661, and the cases on mutual undertakings and bodies exercising self-regulatory powers mentioned at [47] below.
I have written about this previously on the ruling, and about irrationality, however I have previously focused on the absence of of arbitrariness, capriciousness, perversity and irrationality, but looking at the current so-called evidence definition for proscription hearings, I come to the conclusion that  equally important are honesty, good faith and genuineness. I should always have started there!