Baxi Solo 2 40PF sparks but won't ignite during start up sequence?

I have a Baxi Solo 2 40PF and it goes through the start up sequence but sticks on the ignition and just continues to spark continuously without firing up. If I open the inner casing and expose the heat exchanger it immediately fires up and if I switch off and turn on again it goes through the start up sequence and ignites every time!

I checked the air pressure switch and blew into the high section and it clicks okay, which I presume means it is okay. Just in case I replaced it with another and the problem remains the same. I should point out the one I replaced it with wasn't an exact match specification wise.

Any help would be appreciated.

Napoleon Solo
July 2007