How can I fix my Worcester 241 RSF Bolier?

Hi Everyone, I hope you can help me. I've been having problems with my boiler for the past few weeks. I've had 4 British Gas engineers but it has not been fully fixed yet. There is a problem with the ignition. The engineers have changed the circuit, fan, Electrode and the lead. But from time to time the ignition fails. Any ideas what might be the problem?