Everything was working relatively normally until the fan stopped working so I took it out and revived it.
I reinstalled the fan and tested the boiler. The burners fired as normal and everything seemed good..
Until I put the inside cover back at which point it went out and wouldn't relight (Fan still working as normal) Lifted the cover off and the burners lit. Did this several times to prove that I wasn't going mad! It works with the cover loosley fitted so that air can get in but as soon as it's tightened down the burners go off and won't reignite.
I'm wondering if the fan is still not working at full efficiency? Would that make sense? Or am I missing something else?

Phil, March 2011

Cleaning out the venturi solved the problem- just need to replace the worn self-taps and clips now!
Money saved = lots! (Although I will still replace the fan)

Phil, March 2011

I've read elsewhere on this board that the problem could be related to the air pressure switch, I'll check that out and post back if that solves the problem. Meanwhile any other suggestions would be most welcome!

Phil, March 2011