your employer can change your contracted hours and they can do it in a couple of ways.
they can ask you and if you agree then you can change your contracted hours straight away. if you refuse then they can serve notice to you that they are changing your hours. if they serve notice they must give you ample notice i.e,
in the first 12 months of employment no notice is required. after 1 year then the notice is 1 week per every year served up to a maximum of 13 weeks notice. then its either accept the new contracted hours of leave.
this is how things work at my work and i think that it is the norm in the uk but we find that is better to negotiate somthing that benefits both parties rather than have conflicts.
i hope this has been some help.