Thanks, I'll pull the valve and manifold again and remove that burner (and those either side) and see if that does it. If so, I'll let you know. It may be in the morning as the better half has me plowing the garden today.
I decided to take the time to give the burners a shot with the air hose. I blew 150lbs of air into each burner, paying special attention to the the three below the igniter. The heater lit 3 times without failure. So, I'm hoping that was the problem, if not I'll do a thorough cleaning of the burners. Even if it continues to work ok, I will still pull the valve and manifold, remove and clean all the burners to make sure they're all clear.
Thanks in advance for the advice . . . we'll see if it does the trick, I'll update later after I've given it a good (excuse the expression) smoke test.
It's still giving me a problem. If I keep cycling the heater off and on, it will eventually light. Once lit it, will run until reaching temperature but not relight.
Next I'll pull the manifold and the burners and check them for blockage.