UK Car Repair: r reg astra 17 deisel fanbelt replacement, fan belt, pulleys

Question
How do i get the water pump belt off to re fit my new fan belt which snapped

Answer
I do not have details of your model, but usually there is a spring loaded automatic adjuster which has to be retracted.  Did the belt snap because of age?  When the belts are off turn all the pulleys to make sure nothing is siezed or tight. Also change all of the belts.