I have a Mira Sport 7.5kw shower which was installed in 2003. Its recently started to deliver only cold water. It turns on fine and I can regulate the flow as normal. Its initial symptoms were, before it went completely cold, for a few days it cycled between very hot temperatures and very cold on a given flow rate. Now its gone cold. Rather than get a completely new shower I'm hoping to replace a part. Is it likely to be the heater tank or the thermal switch or something else that I need?