Set Appliance State:

Mains Power Off
Gas Off
CH Demand
Boiler Operating Switch in HW and CH position
Heating Control Thermostat (Potentiometer) adjusted to Maximum
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

The pump is situated behind the gas valve and secondary heat exchanger.
Does the pump run?
Note:
Do not take noise or vibration as a sign of pump operation.