how can I tell if the gas valve is being switched?

We have an old Potterton Netaheat 5/10 boiler, everything seems to be working, except gas to the main burner! When either the heating or hot water activates the system, the fan comes on, but the main burner doesn't light. Gas is delivered to the unit as the pilot light is lit.
Based on the wiring diagram can you advise which gas valve, 1 or 2 operates the main burner. Using a Volt-Ohm Meter, how can I determine if the valve is operating based on the voltage across the contacts? One is showing approx 172.0 volts while the other is 246.0 volts.