Wrong diagnosis 2 times

the repair/service department people of ford northcliff are clueless when it comes to diagnosis of a car. Early this year my car was overheating then they replaced water tank costing R4K after a while it was still overheating and the replaced heater and thermostart costing me another R4K. The mechanics promised me that this has been fixed and it won't trouble me again. Funny enough the car still overheat because it was not diagnosed properly. These guys are useless and don't know what they are doing?? I have went to several mechanics and they all said its the water pump that should have replaced in the first place. I just want reimbur*****t on the cost incurred by their incompetence.