"11 [The] test is not comprehensive. The principle is not confined to cases of abuse of trust and confidence. It also includes, for instance, cases where a vulnerable person has been exploited. Indeed, there is no single touchstone for determining whether the principle is applicable. Several expressions have been used in an endeavour to encapsulate the essence: trust and confidence, reliance, dependence or vulnerability on the one hand and ascendancy, domination or control on the other. None of these descriptions is perfect. None is all embracing. Each has its proper place. 12 In CIBC Mortgages plc v Pitt[1994] 1 AC 200 your Lordships' House decided that in cases of undue influence disadvantage is not a necessary ingredient of the cause of action. It is not essential that the transaction should be disadvantageous to the pressurised or influenced person, either in financial terms or in any other way. However, in the nature of things, questions of undue influence will not usually arise, and the exercise of undue influence is unlikely to occur, where the transaction is innocuous. The issue is likely to arise only when, in some respect, the transaction was disadvantageous either from the outset or as matters turned out. ..... 14 Proof that the complainant placed trust and confidence in the other party in relation to the management of the complainant's financial affairs, coupled with a transaction which calls for explanation, will normally be sufficient, failing satisfactory evidence to the contrary, to discharge the burden of proof. On proof of these two matters the stage is set for the court to infer that, in the absence of a satisfactory explanation, the transaction can only have been procured by undue influence. In other words, proof of these two facts is prima facie evidence that the defendant abused the influence he acquired in the parties' relationship. He preferred his own interests. He did not behave fairly to the other. So the evidential burden then shifts to him. It is for him to produce evidence to counter the inference which otherwise should be drawn. ..... 21 As already noted, there are two prerequisites to the evidential shift in the burden of proof from the complainant to the other party. First, that the complainant reposed trust and confidence in the other party, or the other party acquired ascendancy over the complainant. Second, that the transaction is not readily explicable by the relationship of the parties. 22 Lindley LJ summarised this second prerequisite in the leading authority of Allcard v Skinner 36 Ch D 145, where the donor parted with almost all her property. Lindley LJ pointed out that where a gift of a small amount is made to a person standing in a confidential relationship to the donor, some proof of the exercise of the influence of the donee must be given. The mere existence of the influence is not enough. He continued, at p 185 'But if the gift is so large as not to be reasonably accounted for on the ground of friendship, relationship, charity or other ordinary motives on which ordinary men act, the burden is upon the donee to support the gift.' In Bank of Montreal v Stuart[1911] AC 120 , 137 Lord Macnaghten used the phrase 'immoderate and irrational' to describe this concept. ..... 24 ..... It is a necessary limitation upon the width of the first prerequisite. It would be absurd for the law to presume that every gift by a child to a parent, or every transaction between a client and his solicitor or between a patient and his doctor, was brought about by undue influence unless the contrary is affirmatively proved. Such a presumption would be too far-reaching. The law would be out of touch with everyday life if the presumption were to apply to every Christmas or birthday gift by a child to a parent, or to an agreement whereby a client or patient agrees to be responsible for the reasonable fees of his legal or medical adviser. The law would be rightly open to ridicule, for transactions such as these are unexceptionable. They do not suggest that something may be amiss. So something more is needed before the law reverses the burden of proof, something which calls for an explanation. When that something more is present, the greater the disadvantage to the vulnerable person, the more cogent must be the explanation before the presumption will be regarded as rebutted. 25 This was the approach adopted by Lord Scarman in National Westminster Bank plc v Morgan[1985] AC 686 , 703-707. He cited Lindley LJ's observations in Allcard v Skinner 36 Ch D 145, 185, which I have set out above. He noted that whatever the legal character of the transaction, it must constitute a disadvantage sufficiently serious to require evidence to rebut the presumption that in the circumstances of the parties' relationship, it was procured by the exercise of undue influence. Lord Scarman concluded, at p 704: 'the Court of Appeal erred in law in holding that the presumption of undue influence can arise from the evidence of the relationship of the parties without also evidence that the transaction itself was wrongful in that it constituted an advantage taken of the person subjected to the influence which, failing proof to the contrary, was explicable only on the basis that undue influence had been exercised to procure it.'"
"26 Lord Scarman attached the label 'manifest disadvantage' to this second ingredient necessary to raise the presumption. This label has been causing difficulty. It may be apt enough when applied to straightforward transactions such as a substantial gift or a sale at an undervalue. But experience has now shown that this expression can give rise to misunderstanding. The label is being understood and applied in a way which does not accord with the meaning intended by Lord Scarman, its originator."
"220 As to manifest disadvantage, the expression is no more than shorthand for the proposition that the nature and ingredients of the impugned transaction are essential factors in deciding whether the evidential presumption has arisen and in determining the strength of that presumption. It is not a divining-rod by means of which the presence of undue influence in the procuring of a transaction can be identified. It is merely a description of a transaction which cannot be explained by reference to the ordinary motives by which people are accustomed to act."
"21.1 ..... [In the circumstances he knew] he would risk having to live in a caravan ..... for the rest of his life unless the necessary works were commenced [in time] ..... Accordingly, Mr Dowsett plainly had an interest in preserving the planning permission, and if [as was the case on William Macklin’s evidence) the Macklins would only come to his rescue by laying the foundations if he signed the Option Agreement, then he plainly derived a benefit under it."