Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the connector from the warm stat which is situated adjacent to the pump

Measurement:

Measure the resistance between the thermostat and a suitable earth point on the appliance (i.e. the pipe).
Measure the temperature of the flow pipe near the thermostat.
The resistance will vary according to the boiler temperature and it should conform to the table below.
Temperature °CResistance in kohms
30 - 402.1 - 1.3
40 - 501.3 - 0.8
50 - 600.8 - 0.6
60 - 700.6 - 0.4
70 - 800.4 - 0.3