You only have to work overtime if your contract says so.
See Also
Employment Law Rules For Night Shifts | DavidsonMorrisNight Shift Workers UK | Night Working Hours & Rest Period LawsTop 10 tips to get through a 12 hour shiftTaking rest breaks at workEven if it does, by law, you cannot usually be forced to work more than an average of 48 hours per week. You can agree to work longer - but this agreement must be in writing and signed by you.
Unless your contract guarantees you overtime, your employer can stop you from working it.
However, your employer cannot discriminate against anyone, for example by stopping some employees from working overtime while letting others do so.
