Last night my Vulcan Continental floor standing boiler stopped working.

The pilot light is on but will not fire. The Randall 4033 timer is old and
has been slowing down/stopping.

So, I had resorted to manually advancing it to an ON position which has been
fine for about a month.

I have just replaced the timer with a new one and the boiler still fails to
fire.

So, I guess that it was nothing to do with the timer.

The whole system is approx 23 years old.

What else could it be?