Is this the scenario you mean, does it jog anyone's thoughts?
"Employees sometimes argue that what the employer considered a discretionary benefit has become a binding term of their employment contract through custom and practice.
Common scenarios where disputes of this nature may arise are those where the benefit in question is particularly valuable to the employee, such as an enhanced redundancy payment or bonus.
While employees and their representatives might be quick to argue that an implied term has arisen through custom and practice, this is actually much harder to establish than is often believed.
This article examines the legal framework in relation to terms implied by custom and practice and gives some tips for employers seeking to avoid discretionary benefits offered to their employees becoming legally binding."