And found this:
5. How many hours can my employer make me work?
Your employment contract or written statement of terms should tell you your normal working hours. However, as well as providing for annual holidays, the Working Time Regulations say that you should not work more than 48 hours a week on average, unless you have agreed in writing to work more. If you have signed a form saying you will work more than 48 hours a week, you can change your mind and say you no longer want to do this. You may have to give your employer notice of this, but they cannot insist on any more than three months' notice.
The regulation also set out:
the average number of hours a day you can work if you work nights; and
how often you must be allowed to take rest breaks and how long you must be allowed for the breaks.
You can complain to an employment tribunal if your rights under the regulation are broken. You are also protected from being dismissed or treated less favourably for complaining that your rights have been broken.