Employee is questioning management changes to his working hours

An employee who has worked Sunday hours for 4.5 years has been told that this Sunday working is not now a requirement by management i.e we do not require a handyman on Sundays. This has the effect of reducing the weekly hours that he is is used to and he states that we can not do this legally. His contract is for a 40 hour week with additional hours at any other time management deems a requirment. His new rota means that he is maintaining an average of a 48 hour week, His arguement seems to be that we are depriving him of overtime he has come to expect and we have have changed his terms of employment. As we are continuing to give him regular hours above the 40 hour contract (8) I feel we are entitled to reduce additional hours as a business requirment. Does his argument stand

thanks

Tony Farrell

Facilities Manager