Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Thermostat to MAX position
Release the front facia by gently prising up the retaining latches at the top of the facia

Measurement:

Test 1.
Measure resistance between the back of the potentiometer and the L/H connection on the back of the switch.
Expected resistance approximately 0 Ohms.
Test 2.
Repeat test between the back of the potentiometer and the R/H connection on the back of the switch.

Expected resistance approximately 200 Ohms.