I have an Alpha 280E boiler?

The fault I have is intermittent and only happens in the summer when the heating is either switched over to hot water only or the tappets are set at off.
What happens is, when you turn on the hot water the boiler refuses to ignite and the top light comes on, which according to the booklet is caused by a blocked flue.
When you call out an engineer by the time he arrives it has started working again. The engineer checks the flue, can't find a blockage, shrugs his shoulders and says I can't do anything about it unless the fault is active. Give me call if it happens again. Then of course when it happens again by the time he arrives it has stared working again. This has been going for about three summers and umpteen engineers but no one seems to know what to do.