How can i prevent my employer from changing my terms of employment against my wishes?

I have worked for a Public Authority for over 4 years. Previously, weekend working has been on a voluntary basis with payment. Management now want to incorporate this into my contract as a statutory requirement to work ten weekends per anum if needed or unless volunteers are found. I wish the status quo to remain the same. Management state they need to save money on weekend work & yet they propose time off with payment if someone volunteers or time off in Lieu or payment if someone is rostered for weekend work under the contract changes. This would be my first argument - for management to say these proposals will save money is contentious & the wording is spurious & vague. I am prepared to fight this all the way, however out of seventeen people employed in this occupation only two of us are in a union - we are the only ones objecting. During negotiations with the rest of the group( No union involvment) my fellow union member & i were marginalised - we were asked not to attend negotiations for fear we could negatively influence the outcome - in effect we were prevented from debating this with our colleagues. my colleague & i have views on this matter - we wish a twelve month moratorium on this where the voluntary overtime is continued & then reviewed. We have never been offered the chance to state our viewpoints/wishes. The changes in employment terms have been sent through the post to my home address, commencing with change in job title & conditions from the first of June. I am not prepared to accept this & need help as to where to go & what to do from here. I have to say my union seem reluctant to pursue this due to a minority membership. Finally although my non union colleagues have accepted the changes - this is management driven - all the inertia has come from them .