Vurx,
I am beginning to think that the Opel PCV system has become my "raison d'être"! ("reason to be")
The big valve cover hose serves TWO purposes. As Gene has said, it allows crankcase fumes to be vented out from the valve cover, preventing the buildup of pressure in the engine. But this only happens at higher revs, unless the engine is seriously tired, and has significant ring blow-by and/or leaking valve stem seals. In this case, the engine develops more blow-by gas than the smaller hose (which should be routed to the PCV metering orifice on the intake manifold) "sucks" out of the crankcase, and gas fumes flow out from both valve cover ports.
But if you have a reasonably sound engine, at an idle (and high manifold vacuum) the PCV metering orifice sucks more gas volume than the engine creates, and the big hose acts as a "makeup" air supply, keeping the crankcase from being drawn into a vacuum. If the big hose is simply open (and not connected to the inside of the air filter), dirty air is drawn into the valve cover, and in turn into the small PCV hose. So the dirty air enters the combustion process, and this is not a good thing.
Connect the big hose to connector INSIDE the air filter, such as this (the big hose connects to the left side of the tee; I have chosen to connect the charcoal cannister to the right side of the tee, killing two birds with one tee):