“... if the circumstances so demand, a constructive trust can arise some years after the property has been acquired by, and registered in the sole name of, one party who (at the time of the acquisition) was, beyond dispute, the sole beneficial owner: Gissing v. Gissing[1971] AC 886 901 D-E and Bernard v. Josephs[1982] Ch 391 , 404 E-F. But, as those cases show in the absence of an express post-acquisition agreement, a court will be slow to infer from conduct alone that parties intended to vary existing beneficial interests established at the time of acquisition.”