Ive had this issue for a while with mine only lighting when it feels like it.!
I did some analysis work into the boiler which happens to be the super condensing combi from ravenheat. (a great boiler-when it works!) I found the ignition pack which is located at the base to the left on the boiler has a resister which takes a huge energy surge for some reason and goes high resistance causing it to overheat and breakdown down under load. the ignition pack comes as a complete spare and is linked to more than one system on the boiler. NB even though it is manufactured by honeywell they was not prepared to issue one through mail order because of the failure.
Darrel Sutherland, March 2008