Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW Demand
Gain access to PCB (use insulation mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage on the right hand terminal block between the connections marked "sL" and "N".
With a DHW demand mains voltage should be absent.
Note:
Voltage on sL is supplied by the HW OFF
at the cylinder thermostat / timer.