"Tom" wrote
Thanks. One thing I hadn't mentioned was that there was no smell of oil
after
the failure to ignite incident. (I did swivel open the hatch at the time,
and
I'd suppose it'd take a while for fuel oil to evaporate from the closed
chamber up the chimney.) So I'd guess that tends to argue against lack of
spark.

It may not smell, especially if there was a time lag before you noticed it.
If it happens again, just reach in and rub the electrode with your finger
and it may start. All it takes is a hairline piece of carbon from tip to
ground to stop it from firing.