 After that, they consulted their solicitors and sued for the balance.Lord Denning MR held that the doctrine of part payment of a debt not discharging the whole ‘has come under heavy fire’ but noted that estoppel, deriving from the principle laid down in "Hughes v Metropolitan Railway Co.", could give relief in equity.Although in his opinion part payment of debt could satisfy a whole debt, he found that Mrs Rees had effectively held the builders to ransom.