Basic rule: Past consideration is NO CONSIDERATION! This means that if a task is already completed and then a later promise to pay is made, that promise is unenforceable as the consideration was in the past.
Case: "Roscorla v Thomas". In this case there was a sale of a horse, the later promise was that the horse was sound and free from vice. This was far from the case... unenforceable.
This case took place before the Pre-Judicature Act and was ... confirmed as good law in: "Re McArdle". In this case Mrs McArdle spent a vast amount of money on repairs and decoration on her house. The promise was that the children would pay after the mother died. She died and children refused to pay ... unenforceable.
AO2: This has been classed as being unfair as promises can be broken ... the law has created an exception. If there is a request for a service which carries with it an implied promise (promise to pay) then a later promise is "enforceable".
Cases: "Lampleigh v Braithwaite". In this case a request was made to get a pardon from the king. There was a promise to pay £100 but Braithwaite refused to pay ... it was implied that some money would be paid. This was confirmed as good law in "Pao On v Lau Yiu Long".