Pilot light is lit but wont ignite when water/heating turned on?

We have a Corvec 45BF Chaffoteaus et Maury boiler which has been serviced in the last six months and although it is old, we were informed that it still had at least another 3yrs life left in it. Unfortunately it will only intermitently 'fire up' whether its just set for the water or both water and heating. The pilot light is on by the way! I thought it mightve been a problem with the timer switch, but have since been told that this is not the problem!!! Please help as its getting rather cold!!!!!!