If it is a high mileage car and no history of water pump change I would do it with the cambelt. It just makes sense to me to do both. The impeller gets corroded by the water and can cause cavitation which can cause vapor to form. This will make the cooling system less efficient. My last cambelt change was <£400, i did not change the waterpump as the car only had 30K, I wish I had now though.