ADD THE FOLLOWING AT THE BEGINNING OF THE PARAGRAPH: There are only limited circumstances in which a transaction may be challenged in equity on the grounds of mistake. Unilateral transactions may be challenged on the grounds of mistake, whether the cause of the mistake was fraud, undue influence, misrepresentation or simple mistake: but where the basis of challenging the transaction is simple mistake, it is necessary to establish that the transaction was not merely unilateral but voluntary [NOTE 1A], and that the mistake was of so serious a character as to render it unjust on the part of the donee to retain the property [NOTE 1B]. It has been held that the mistake must be a mistake as to the effect of the transaction itself and not merely to its consequences or the advantages to be gained by entering into it [NOTE 1C]: if this is correct [NOTE 1D], then a misapprehension as to the consequences (including the tax consequences) of a unilateral transaction will not be relevant [NOTE1E].  The position is different in respect of other transactions.

ADD NEW NOTE 1A: See Smithson v Hamilton [2007] EWHC 2900 (Ch), applying Gibbon v Mitchell [1990] 1 W.L.R. 1304.  A suggestion that the principle has a wider application (supported by dicta in A.M.P. v Barker [2001] P.L.R. 77 and Gallagher Ltd v Gallagher Pensions Ltd [2005] EWHC 42 (Ch)) was rejected on the basis that those dicta had been superseded by Great Peace Shipping Ltd v Tsavlaris Salvage (International) Ltd [2002] EWCA Civ 1407:  see [2007] EWHC 2900 (Ch), at [110]-[119].

ADD NEW NOTE 1B: See Ogilvie v. Littleboy (1897) 13 T.L.R. 399, at 400 per Lindley LJ; affirmed sub nom. Ogilvie v. Allen (1899) 15 T.L.R. 294.  See also Sieff v. Fox [2005] EWHC 1312 (Ch), paras.[99]-[100].

ADD NEW NOTE 1C: See Gibbon v. Mitchell [1990] 1 W.L.R. 1304, at 1309 per Millett J; also Anker-Petersen v. Christiensen [2002] W.T.L.R. 313 and Wolff v. Wolff [2004] EWHC 2110 (Ch).  Since the effect of a transaction includes its indirect effect (see Ellis v. Ellis (1909) 26 T.L.R. 166), the distinction between effect and consequences may not be an easy one to draw:  see Sieff v. Fox [2005] EWHC 1312 (Ch) at paras.[95], [102] and [104]-[105].

ADD NEW NOTE 1D: The scope of the proposition is uncertain since, as explained by Lloyd LJ in Sieff v. Fox [2005] EWHC 1312 (Ch), there is no decided case directly on point:  loc cit, para.[106]. The proposition does not apply where a trustee mistakenly disposes of trust assets, since that case is governed by the rule in Re Hastings-Bass:  see para.9-09 ff above.  In Sieff v. Fox, although expressing some doubts about the correctness  of the proposition, Lloyd LJ was content to proceed on the basis that the proposition applied in cases where the transaction is by an individual disposing of his/her own assets:  ibid, at paras.[106] and [108].  See also (2006) 122 L.Q.R. 35 (Mitchell).

ADD NEW NOTE 1E:  See Anker-Petersen v. Christiensen [2002] W.T.L.R. 313, at 330-31 per Davis J.

NOTE 2:  ADD:  For the view that there may still be an equitable jurisdiction to grant rescission on the basis of a unilateral (as opposed to a common) mistake, see [2005] L.Q.R. 393 (Yeo).