The general position in law is an assurance to do something is not enforceable without the person receiving something in exchange. The court requires consideration to enforce a promise. The exception to this is estoppel, which is where it would be unfair not to enforce a clear promise. That applies where the person who received the promise has relied on that promise to their detriment.
The question then becomes what is sufficient to amount to an adverse reliance? The courts at first said that it was an all or nothing test. However, in cases where there had been some, but not much, reliance the courts felt unhappy giving all or nothing.
The position was complicated by the court saying that the detriment has to be significant, otherwise the promise is not fully enforceable. In those circumstances only part of the promise might be enforced by the court. How much a person might receive is very much at the discretion of the court. This lead to a great deal of uncertainty as to how much a promise was worth.
Recent case law, such as Bradley v Taylor  EWCA CIV 1208 has tried to put back some certainty. There they confirmed that if there is any significant detriment then the normal position is the court will enforce the whole promise, not half the promise.
Another recent case went down similar lines. There Mrs Lothian, an elderly lady who ran a seaside hotel, made a promise to her cousin. When she became very ill she asked her cousin to look after her, promising that she would leave the hotel to her. Her other cousin, who had previously been left half her estate in a will, objected after Mrs Lothian’s death. The matter came to court in November 2014 in the case of Lothian v Dixon and Webb. The Judge held that in that case Mrs Lothian had acted to her detriment, in giving up her life in Scotland to come down and look after her cousin. The argument that there was a counter balancing benefit in the form of free lodgings and board was rejected.
This goes to show that the court will look at detriment in the round and is generally inclined to enforce a promise, where the person relying on the promise has helped the deceased in her last years.