Set Appliance State:

Mains Power Off
Gas Off
Turn Hot Water Tap On
Rotary Switch in DHW position
Keep Boiler Case Switch depressed
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Does the fan operate at its high speed?
Note:
When fan runs on high speed this proves DHW demand circuitry.