The fault will almost certainly be the seal around the combustion box door, remove case and inner door panel, you should now be able access combustion door, it has the electodes attached to it, remove the door and replace the seal around the outside of the inner face.
Once you have done this ensure that combustion door is tight when you re-fit it, reassemble outer door and casing and you should be fine.
Regards
Kev the boiler engineer
March 2006