What is the legal test for undue influence?

Both parties rely on Wingrove v. Wingrove (1885) 11 P.D 81 at pp. 82-83 for an authoritative statement of the operative level of coercion to constitute undue influence: To be undue influence in the eye of the law there must be - to sum it up in a word - coercion. It must not be a case in which a person has been induced by means such as I have suggested to you to come to a conclusion that he or she will make a will in a particular person’s favour, because if the testator has only been persuaded or induced by considerations which you may condemn, really and truly to intend to give his property to another, though you may disapprove of the act yet it is strictly legitimate in the sense of its being legal. It is only when the will of the person who becomes a testator is coerced into doing that which he or she does not desire to do, that it is undue influence. The coercion may of course be of different kinds, it may be in the grossest form, such as actual confinement or violence, or a person in the last days or hours of life may have become so weak and feeble, that very little pressure will be sufficient to bring about the desired result and it may even be, that the mere talking to him at that stage of illness and pressing something upon him may so fatigue the brain, that the sick person may be induced for quietness’s sake, to do anything. This would equally be coercion, though not actual violence.