A contract of employment is a legal agreement between the employer and the employee. It contains terms, either 'express' or 'implied', that cannot lawfully be changed or varied without further agreement between you. Where there is a recognised trade union, this further agreement should be negotiated through collective bargaining.
If the employer wants to change an important contract term, it should first carry out a proper consultation, explaining the reason for the change, and allowing time for staff to consider the proposal and to suggest alternative ways of achieving the same result. For example, if the employer wants to cut pay or benefits, employees might want to suggest other ways of achieving the same savings.
Employers should offer compensation for any loss resulting from any change, and give enough advance notice so that employees are able to prepare.
Employees can decide to accept a change, and many contract terms are of course varied from time to time by mutual consent, for example a pay rise.
Some contracts contain a written term, apparently permitting the employer to make unilateral changes (i.e. without your agreement). For example, it might say something like "changes can be made to these contract terms without your consent to reflect the changing needs of the business".
Even if your contract contains this kind of clause, any changes must be reasonable and should only be made after consultation. Your employer must not breach the contractual duties of mutual trust and confidence and good faith that is owed to all employees, and must not discriminate.
Your employer should not use this kind of term to force through significant negative changes to your job without your agreement.
Speak to your union rep if you're a union member, as union solicitors may be able to provide advice as to what changes the employer is allowed to make by relying on this kind of term.
Often a well-organised collective approach is the only way of persuading the employer to change its mind.
Finally, remember that where changes are made to your contract, employers must give you written notification of the change within one month.