hot water in header tank

[shaka]

Hi hot water getting into header tank only immersion heater fitted, no central heating fitted new thermostat fitted did not solve problem
There is a pump fitted to hot water to boost to shower cold in shower mains cold. pump been fitted for about 2 years first time this problem has occured thanks for looking brian

 

[seco services]

More likely cold mains passing to hot gravity.
Either the shower or mixer tap with gravity hot cold mains.

Isolate each one in turn to locate the faulty one.