Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Ensure appliance is cool
Remove the right hand side panel
Press the reset button on the high limit thermostat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset the appliance by:
Turning the selector switch from CH/DHW position to the OFF/Reset position, wait a few seconds then turn the selector switch back to the CH/DHW position.
Apply a DHW demand.
Has the boiler reset?
Note:
It is important to apply a DHW demand after resetting.