First, remember that you have a legal right to receive a written statement of particulars of your main terms and conditions of service, and for that document to be kept up to date. You must be given this document on or before your first day at work. And if changes are made, you need to be given a revised statement within one month of the change.
In practice, most disputes over terms and conditions of employment are best addressed collectively through negotiation, where more than one worker is affected. Speak to your union if one is recognised at your workplace. If there is no union where you work, you might think about organising support for union recognition. Use our Union Finder tool to find a union that operates in your sector.
If you would rather deal with the problem on an individual level, or if you are the only one affected, speak with your immediate supervisor in the first instance. If you do not receive a satisfactory answer within a reasonable amount of time, raise the issue with a more senior manager.
If you are not satisfied with the answer from management or you do not receive one, you may wish to take out a formal grievance in accordance with the provisions of your organisation's grievance procedure.
The Acas Code Of Practice on Disciplinary and Grievance Procedures (PDF, 167KB) provides guidance on how employers should address grievances.