In the care home where i work as electrician we have a boiler as above and for a long time it gives a loud report on ignition[woomf sort of sound] casing shakes.
serviced every year,new p c b last year. was told it could be gasket on front casing but present one looks fine. could it be that main gas burner is on at same time as spark? should pilot be on seconds before main burner?

Neil, November 2011

many thanks to Jonney and Bill. Most helpfull

neil, November 2011

That is caused by delayed ignition; the ignition electrode is first place to look, it might need cleaning or the spark gap might need adjusting ( should be approx 4mm on that blr). Secondly, verify the gas pressure setting at the GV outlet as it might need adjusting.

Jonny, November 2011

Could be dirty ignition electrode, wrong gap on electrode, or even the gas valve needs adjusting/setting up properly

bill, November 2011