This depends on your contract of employment and the specific terms within it relating to hours of work.
You may find that your employer has included some clause relating to reasonable requests from management, they may use this in their deffence.
I think more details could be needed to answer this question more fully. Or you could seek advice from a Union if you are a member or a Solicitor.
With regards to getting yourself heard, do you work in a small or big company? You should tell your line manager in the first instance that you are unhappy, then possibly speak to you HR department or rep if you have one.
You can even follow the governments guidlines in raising a formal grevience with your employer if you get no joy.