[Closed] Yet another boiler problem!

I know there are a few heating engineer types on here so this is my problem:
Saunier Duval combi, approx 4 years old. Hot water works perfectly but heating doesn't. The pump is running, there is pressure in the system and i've bled the top rad with the system off then topped it up again but the boiler won't fire up. Sometimes it does but only for a few seconds, its like it decides it hot enough and shuts down. So basically its making all the normal noises (pump running etc) but the gas isn't firing up for the heating bit.
Any clues or anything i can do till i'm forced to ring the man on monday?
I have a coal fire going so i'm not going to freeze!