This isnt rearly a legal issue mate - there has not been violation of the law.

Your contract with them states your manager will agree your days - This told him you could work Saturday befor, so he gave you Saturday.

You could always ask to be moved to another night, but ultimately your at the mercy of the manager

Remember, assuming your contract was signed relativity recently, your probably still in your probation period - they could easily fire you without cause if you refuse to work the days they want you to work.

Going through point by point: -Do I have to work saturday - yes, if thats what your manager is telling you he needs you to do - if he refuses to change days, then you'l have to accept it or leave. You could always try and argue out of it, but if they hired you cause they need a Saturday night person, youl be out of luck --you keeping your end of the contract - technically you havnt - you said you would be flexible - if your refusing to do it, your not being flexible.

(I know you think your being flexible by offering to work other days, but that's not flexible - that's you offering alternatives that suit you -flexible is doing things they want you to do)

-Sueing them - not a chance in hell. You have no claim. Even if you did, Employment tribunals cost money. They will have a legal department, you would have to get a solicitor, and they would definitely fire you if you sued them

What can you do? - Try and talk manager round, accept days or leave.

(I get that it sucks - From 16-18 I worked friday and saturday nights at sainsburys, and a sunday, doing a 16 hour part time gig - generally its not as bad as you think - Saturday evenings slowish work compared to most week nights, and if you ask in advance you can get probs gets nights off - other part time staff members who work during the week evenings may trade you days.)