What's this talc technique? Simply put some talc on air pipes and see if it blows it back up?Try the talc thing on the air intake side and although I’ve not had a engine apart the fact you said it was drawing air in through the cam bolts doesn’t seem right. I’m guessing by now the warranty they gave you when they rebuilt the engine has long since gone!
Definitely keep the faith. you may want to also post on the vauxhall owners network forum, in the Corsa D section. It is little more active so may get a few more ideas.I will give that a go see what happens, and yes the warrenty is out now sooner or later it will get sorted just hope sooner.