Some feedback,
What has the 3way diverter to do with not firing? 3 way diverter, switches between Heating and hot water., On a combi, it would be within the unit, On others as the back boiler you have, It is usually near to the Pump unit, Or can't he find that either. If he cannot see it, Maybe he needs a picture. YOU will NOT need a new boiler, New Control, Maybe, More checks need to be done. If you google your model, as well as finding this post, you will find other information.
Sheepdog, March 2011