My sime super 90 has developed a dely in igniting?

I have a similar problem with a Sime Halstead 30/90 TRIO, however something is preventing the burners from firing up (at all), I can see that the piolt light is on, and the burners flicker and attempt to light without fully firing up.... can anyone please advise on this, as I have already had a certified boiler guy out to check - but no luck. Thanks very much.

SG
August 2011

sounds like a sticking air pressre switch

Ian
July 2011