Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U-gauge to inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Monitor the inlet pressure throughout the boiler's operation.
Does the pressure remain above 18mb (37mb for LPG)?
Note:
When "U" gauge is removed test point should be tightened
and checked for gas tightness.