Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Remove electrical connector from gas valve and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset appliance if required
(press reset button for 2 seconds and release).
Measure for DC voltage across pin 9 (green lead) connector X20 and a suitable earth connection during the sparking stage of the ignition cycle.
Expect approximately 22 Vdc.
This will be repeated 5 times before lockout occurs, reset as required until test complete.
Does the gas valve voltage behave as described?