stopped working? can mean many things, no water? try the pressure switch inside, it is designed that when pressure falls below a certain level it switches off to protect the heater, if no heat, then the overheat stat may be faulty, No Power? ( electricity )
check that the switch is on, either a pull cord or a switch outside the bathroom, also check that the switch is on in the fuseboard. If it is off, then beware you may of switched itself off due to a prob, A Triton T80 is a rather old shower, consider replacing, but again beware that the cable supplying the shower will need to be upgraded to a 10mm cable.
Will Stirling
May 2009