Did the plumber check the valves in the appliances?

Ripped: No, the plumber did not check the valves. I'm not too pleased with the thoroughness of his detective work.

Did they smell unburnt NG or possibly combustion fumes that might indicate a blockage in the vent?

Your provider may be able to do a test, I live in California, the utility company does this.

I don't understand why you have seemingly not reported this to your Gas Company???

Wow!

Update: I had to leave the house for a week (unrelated) and when I got back, I called the gas company. They sent someone out immediately and he checked everything with a very sensitive gas detector. He found a very small leak in the input gas valve (the one that's activated by the thermostat), and recommended replacing it. So I called my boiler maintenance man and he came and replaced it and, for good measure, replaced a few other components, such as the thermocouple. It cost ~$900.

This is what you should have done originally, as I said!