The only way to try and make the employer change your contract and make the 39 hours permanent is to rely on the law of custom and practice.
There is a principle in employment law where terms may become implied into an employment contract by ‘custom and practice’. 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 an implied term. Nevertheless, it does not prevent employees from directly raising this argument with their employers.
The basic requirement for implying terms is the presumed intention of the parties, in other words - did the employer and employee intend for the terms in question to be treated as contractual. In general, a practice would need to have been clearly communicated and consistently applied for a substantial period of time before it can be considered an implied contractual term. Therefore, something that is uncertain, not communicated properly, not been applied consistently or has just been around for a few months is unlikely to qualify.
You have to be mindful that just because you have done these hours for around a year does not necessarily make it long enough. A court may decide that it needs to be longer, a few years for example. Saying that if certain principles are met they could still qualify a practice which has only been around for a year.
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