Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Ensure boiler is not in an overheat state
Disconnect the brown and blue leads from the CH limit thermostat (thermostat which is furthermost forward of the two thermostats on the R/H side of the heat exchanger)
Ensure boiler is not in an overheat state
Disconnect the brown and blue leads from the CH limit thermostat (thermostat which is furthermost forward of the two thermostats on the R/H side of the heat exchanger)
Measurement:
Measure the resistance across the exposed terminals of the thermostat.
Isolate as per TOPs P4
and refit as necessary