Despite the lack of a definitive formulation, it is widely accepted that the elements of assurance, reliance and detriment must be present in order to found a claim of proprietary estoppel . The doctrine has however been widely criticised for being too flexible and uncertain. The main cause of this uncertainty is the lack of clarity surrounding the role of unconscionability. It has been stated that unconscionability is “at the heart of the doctrine,” and yet there is “little guidance as to what it means, little explanation of why it is at the centre and thus virtually no consideration of the role it might play in providing both a justification for, and a limitation on, successful estoppels” . Commentators have largely agreed that there is a “need to develop clear parameters for the operation of the doctrine, else it really will be a discretionary panacea for all ills whose application is unpredictable and uncertain.
Prior to Thorner v Major  and Cobbe v Yeoman’s Row Management Ltd  it had been 142 years since a case of proprietary estoppel had reached the House of Lords. Therefore it was hoped that these cases would give the judiciary a long awaited opportunity to clarify the doctrine.
In Cobbe Lord Walker stated that “Equitable estoppel…is not a sort of joker or wild card to be used whenever the court disapproves of the conduct of a litigant who seems to have the law on his side. Flexible though it is, the doctrine must be formulated and applied in a disciplined and principled way.”
The House of Lords appeared determined to address the criticisms and it was hoped that they were about to define and clarify the doctrine, especially the role of unconscionability.
Lord Scott, who gave the leading judgement, stated that unconscionability alone is never enough to found a claim of proprietary estoppel. “To treat a ‘proprietary estoppel equity’ as requiring neither a proprietary claim by the claimant nor an estoppel against the defendant but simply unconscionable behaviour is, in my respectful opinion, a recipe for confusion”.
Lord Walker addressed the uncertainty over whether unconscionability is a separate element, by stating that “unconscionability… does in my opinion play a very important part in the doctrine of equitable estoppel, in unifying and confirming, as it were, the other elements. If the other elements appear to be present but the result does not shock the conscience of the court, the analysis needs to be looked at again” .
These emphatic statements confirmed that unconscionability alone is insufficient to give rise to a claim of proprietary estoppel and that the traditional elements must always be present. It is suggested however that these pronouncements did very little in practice to define or clarify the concept.
The modern approach to proprietary estoppel which began with Taylor Fashions Ltd v Liverpool Victoria Trustees Co Ltd  adopted a “broad test of whether in the circumstances the conduct complained of is unconscionable without the necessity of forcing those incumbrances into a Procrustean bed constructed from some unalterable criteria.” Despite this statement the judgement in Taylor Fashions did not rely on...