burner wont stay on?

Check the simple things first! The fault finding chart in the manual is very basic and could lead you to a wrong (and expensive) diagnosis. Here is a summary of my experience with this boiler.
a. Pilot not lighting - caused by sticking fan bearings. Lubricate with penetrating oil. Fan should spin easily to activate pressure switch by creating a depression in the combustion chamber. This starts the ignition sequence (assuming the pressure switch is ok)
b. Main burner not lighting, lighting intermittently or tripping - pilot flame too small due to either partially blocked pilot injector or pilot supply pipe. The injector has two very small holes which can be cleaned with fine wire. The pipe can be easily removed and checked by blowing through and/or clearing with wire. The pilot must heat the ignition electrode sufficiently to generate the very small electric current which is input to the ignition board activating the main burner gas valve. This signal can easily be leaked to earth from the connecting wire where it is connected in parallel to the condensate trap sensor by a screw down terminal at the back of the boiler. Dirt can accumulate here causing this problem. To prove this, disconnect the wire from the sensor if the above checks do not fix the problem. If main burner is still not firing and staying on (should light within a second or so of pilot lighting) renew the ignition electrode - costs less than a tenner.
I have owned this boiler for seven years and have never had to replace expensive items such as fans, circuit boards or gas valves although I am not impressed with the reliability. Good luck.

Ravbob
March 2011