Thank you. As far as the contract is concerned, you have no guarantee on which days of the week you would be working and there is also a flexibility clause, which means the employer could ask you to work any day and there is no guarantee of you working any set days. This is the starting position at least. Also there is no obligation on them to assign you the days so that you can go and do another job - they do not have to take that into account and can only think about what suits them, not what suits you because you have other working commitments.
Saying that, 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 tribunals 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 in any negotiations.
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. Therefore, something that is uncertain, not widely communicated or applied consistently or has just been around for a few months is unlikely to qualify.
In your case you can try and argue that the fact you have worked these days consistently for 3-4 years means they could have become an implied contractual term.
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. 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.
However, if they refuse to accept this argument and you cannot resolve the issue internally, you either have to accept the situation or resign and make a claim for constructive dismissal in the employment tribunal.
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