Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Boiler ON/OFF Switch OFF
CH Temperature Selector OFF
DHW Temperature Selector OFF
Disconnect leads from Modulating Solenoid (blue leads)

Measurement:

Measure resistance across the exposed terminals on the Modulating Solenoid.
For gas valve A resistance should be approximately 25Ohms.
For gas valve B resistance should be approximately 20Ohms
Reconnect leads after test.