[12] The "single document" requirement is strictly applied: Lord Justice Rimer observed in Keay & Anor v Morris Homes (West Midlands) Ltd. (2012) regarding section 2(1) that:... Its effect is merciless.

As Lord Scott of Foscote stated in his speech: 29.... proprietary estoppel cannot be prayed in aid in order to render enforceable an agreement that statute has declared to be void.

The proposition that an owner of land can be estopped from asserting that an agreement is void for want of compliance with the requirements of section 2 is, in my opinion, unacceptable.

Equity can surely not contradict the statute....This mirrors the observation that "The doctrine of estoppel may not be invoked to render valid a transaction which the legislature has, on grounds of general public policy, enacted is to be invalid,"[18] which has been cited in other cases in the matter by the Court of Appeal.

[19][20] The constructive trust remedy that is available under s. 2(5) of the Act, however, operates under principles distinct from those of estoppel, which can lead to problems in application and enforcement.