I'm pretty sure this has come up before but can't find the thread. John is now fiddling with our thermostat on the shower and in danger of scalding himself. Is there a lock you can put over the temperature gauge?
For reference HSE say
50[SUP]0[/SUP]C severe scalding
43[SUP]0[/SUP]C recommended maximum shower temperature
60[SUP]0[/SUP]C recommended min temperature to kill salmonella, so keep run after mixer valve short.
The answer may be to have a plumber fit an in-line thermostatic valve somewhere in the pipe work. This would control the maximum hot water temperature before reaching the shower.
Otherwise turn down the hot water, at the boiler/hot water tank, this would affect the whole house though.