I have got a 2007 1.8 petrol wanting to change the fan belt how much would it be for a genuine one thanks

 

by fan belt you mean the serpentine (accessory drive) belt? Check for vids on Youtube on what bits need to be moved (not sure how your model is laid out but you may need to move one of the fluid reservoirs etc) and there's a tensioner that needs a tool to release the tension which allows the belt to slide off the idlers. As a best practice you should feel for play/stiffness on the idlers as well as the tensioner when the belt is off.