Kevin,
I had same problem 3 -4 years ago, changed the thermister but also found that the relay at the front of the board had come unsoldered due to the transformer heating it up. This relay needs to be on to kick the fan in which in turns triggers the air switch thus allowing the ignition. This also has some effect on the heating but as the cycling is every minute or so you don't realise.
Problem I have at moment is that all the bits to do with the air switch are 10 years old and become brittle - if you are testing the switch be careful not to break the hosetails on the fan as I have - no plumbers shops open Boxing day so blankets all round.
Hope this helps too
Cheers
Martin
Martin
December 2007