Set Appliance State:
Set Multimeter to mVolts DC 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Set multimeter to measure DC 300 mV range.
Clip -ve lead to bottom connection of Klixon Overheat.
Clip +ve lead to outer casing of thermocouple.
Hold in ON Button, pilot lights.
The voltage will increase slowly up to around 14mV DC
after approx 20 seconds.
If the thermocouple is faulty (open circuit) a voltage of
approx 25mV DC will be observed.
Clip -ve lead to bottom connection of Klixon Overheat.
Clip +ve lead to outer casing of thermocouple.
Hold in ON Button, pilot lights.
The voltage will increase slowly up to around 14mV DC
after approx 20 seconds.
If the thermocouple is faulty (open circuit) a voltage of
approx 25mV DC will be observed.
Isolate as per TOPs P4
and refit as necessary