Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
ON/OFF Switch in ON Position
CH Temp Selector in Minimum position
DHW Temp Selector in Maximum position
Mode Selector Switch in Heat and Hot water position
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:

Allow Fan to run and ignition sequence to commence.
Does the U-gauge show a constant Inlet Pressure of at least 18mb throughout the ignition sequence?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.