Unhappy consumer

Hi Peter

I have a leaking geyser that was bought 2 years ago. The cylinder has a 5 year guarantee. They sent their plumbers Everstream plumbing and they voided the guarantee because we have a stop **** in the house which they claim it caused the leakage. I don't understand how so as most house have stop **** for water supplies in the house. The geyser has its own stopper for incase there is a need for switching water off. I am not happy with the report at all I think they are taking advantage of me. Could you kindly assist with this regard.