At 12yrs old this would make it 99-2000 and one of the last Carvers, in which case, the electric water heating is protected by an over temperature thermostat, if it overheats, a red button on the inboard end of the tank needs to be pressed to reset it.
However on these last Carver heaters they covered the button with a flap, heaven knows why?, but it's certainly not obvious to see. you will though see the white plastic box sticking out of the insulation on the end of the tank, the flap is on the cover plate.