Check if your employer can make changes to your contract
If your employer wants to change a term in your contract, this is called a 'variation of contract'.
Your employer should only make a change to your contract if at least one of these applies:
you agree to the change
your contract says your employer can make certain changes - this is called a 'variation clause'
the law is changing - for example if you get the National Minimum Wage and the rate changes
If you get a new employer because the company is sold, or because a service you work in transfers to a new employer, this is called a TUPE (Transfer of Undertakings Protection of Employment)...