Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Check the continuity of the High Limit Thermostat - remove the thermocouple connection from the interrupter block on the gas valve and place one probe on the interrupter block connection.
Disconnect the High Limit Thermostat from the rear of the gas valve.
Test 1 :
Test for continuity/zero resistance across the High Limit Thermostat flying leads.
Test 2 :
Leaving the probe on the interupter block connection put the other probe to the pipework near the thermostat.
Expect the reading to be open circuit.
Note:
Make sure you make good contact with the probes, any resistance on test 2 can cause the interuptor to fail.
Ensure that the boiler is cool before carrying out the test.
On replacement check that the component connections are clean and securely made.
Disconnect the High Limit Thermostat from the rear of the gas valve.
Test 1 :
Test for continuity/zero resistance across the High Limit Thermostat flying leads.
Test 2 :
Leaving the probe on the interupter block connection put the other probe to the pipework near the thermostat.
Expect the reading to be open circuit.
Note:
Make sure you make good contact with the probes, any resistance on test 2 can cause the interuptor to fail.
Ensure that the boiler is cool before carrying out the test.
On replacement check that the component connections are clean and securely made.
Isolate as per TOPs P4
and refit as necessary