Boiler Gas Valve Solenoid

I'll know shortly if my gas valve/operator is the cause of my boiler not firing up. It's on an old Baxi Bermuda 551. The boiler pilot light is lit.

Q1: If the valve's ok, and it's just the operator that's died, can I replace it without disturbing the gas path? (Put another way, does removing the operator let the gas out?)

Q2: How to set the adjuster after remove/replace?

I did a quick Ohms test on the thermostat: Off = open circuit. On = about 1 Ohm (regardless of setting).

I'll check if power's getting to the valve.

Thanks

Edwin Bath.