What exactly is the wording of your contract re: hours? It's incredibly common to have a '12 hour contract' or whatever but unless it says 12 hours max/21 hours max I would be wary of challenging it. My contract said: 'Your working week will be 15 hours, 2 evenings and 2 weekend days' I generally worked about 22 hours and was able to work 37.5 hours on several occassions. So unless yours says maximum in it somewhere that it is up to the needs of the employer what hours you work AS LONG AS your hours are over 21 hours. Did your contract change in any way when you moved branch?
Edit: Just saw that your contract does not mention hours. I wouldn't think you have a leg to stand on in that case.