Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamp to DHW outlet pipe underneath boiler (see picture)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow boiler to run for hot water and temperature to stabilise.
Compare the water temperature at the DHW outlet pipe at the boiler to the temperature at the tap.
Are the temperatures approximately the same, or is the temperature at the tap significantly lower?