Many thanks for your patience, it is appreciated. I am now pleased to be able to provide further assistance with your query. First of all, I am sorry to hear about the issues brought up by this. It must be a frustrating situation to be going through.
There is a principle in law known as ‘custom and practice’, under which certain terms may become implied into an employment contract. This makes them contractually binding even if they are not written down anywhere. This area of law is rather complex and it is usually only down to the courts to establish with certainty if something had become such an implied term. Nevertheless, it does not prevent employees from directly raising this argument with their employers in any negotiations and trying to pursue it further with them.
From a legal perspective, to become an implied term a practice must be "reasonable, notorious and certain". In simpler terms this means it must be well established over a period of time, known to employees and clear and unambiguous. So it would need to have been clearly communicated to affected staff so that they have a degree of expectation for it and then it must have been consistently applied for a substantial period of time. Therefore, something that is uncertain, not widely communicated or applied consistently or has just been around for a few months is unlikely to qualify.
Case law has suggested that the following are important factors when considering whether a term has become implied into a contract:
- On how many occasions, and over how long a period, the terms in question have been applied - the more times they have been applied and the longer the period over which this has occurred, the stronger the argument they had become implied into the contract
- Whether the terms are always the same - large differences will make the argument they had become implied weaker. Again, consistency is key
- The extent to which the terms are publicised generally to the affected staff - there must be widespread knowledge and understanding amongst the affected workers that such terms were being applied
Whilst the argument of custom and practice can be raised with the employer in negotiations, they could of course refuse to accept it and if that is the case it can only realistically be challenged by taking this to an Employment Tribunal for formal resolution. Before that option is pursued, it may also be worth raising a formal grievance to give the employer one last chance to resolve this internally.
So for now you can argue that the way you have been paid over these years has created a contractual precedent through custom and practice and pursue it with the employer that way.