Baxi heater won't fire up from cold?

We have a Baxi boiler which has developed a fault which we can't seem to fix. The pilot light won't ignite when the system is cold. If you loosen the seal on the burner box to let some oxygen in then it fires up straight away and runs fine until the heating goes off, but then won't fire the next morning when cold. We've changed the fan, the gas detector, added a spacer behind the fan and had it serviced. Our heating engineer doesn't know what else it can be - any suggestions?