Help!!! Employer forcing overtime on a 30 hr contract

Hey I'm on a 30 hr contract but work 8 hrs everyday, which I'm rota for. This is assuming I'm willing to do it but there is no choice. Its a warehouse for a large national company and I understand that orders need to be picked but why is the overtime compulsary? When ask to go at end of rota shift they say no its gonna be another 2/3 hrs or sometimes an hour. There used to be a late shift but they took it away. So is it okay to demand to go after rota shift of 8hrs? Or can they sack me? Contract is so open it can be interpreted In many ways. What can I say to them as I'm not prepared to leave at 9,and come back at 10 because they changed the way the shifts work. Can someone help

Help!!! Employer forcing overtime on a 30 hr contract