Emily Bueno, Associate in the Private Wealth Disputes Team, wrote an article on Proprietary estoppel for ThoughtLeaders4 Private Client Magazine.
In the article she discusses the doctrine, which "despite having a jargonistic name, is a common sense doctrine, developed in recognition of the fact that people do not live their lives in ways that lawyers advise – that is, by formalising agreements with the assistance of qualified professionals. It recognises that where there has been (a) an assurance of sufficient clarity in relation to the assuror’s property (b) reliance by the claimant on that assurance and (c) detriment to the claimant in consequence of her reasonable reliance then an “equity” in favour of the claimant arises."
You can find the full piece at page 31, here.