You don't say which Doblo. Petrol or Diesel and engine size.
If diesel then the pump can be replaced without disturbing the cam belt. Basically you use two bolts to secure the pump pulley in position and withdraw the pump from the back.
However I would be inclined correctly lock down the flywheel and cam shaft before proceeding just in case something goes wrong.
If you are not that far away from a cam belt change then it might be better to do the belts, water pump and fuel pump at the same time.