I now know the function of the one-way valve that was replaced yesterday, so shall share the information here.
Nasty gasses build up in the crankcase. Instead of venting to the atmosphere these are fed into the inlet manifold where they can be burnt. I guess this is more environmentally friendly.
In a normally aspirated car this is fine since the manifold is always below atmospheric pressure. However in a turbocharged car the manifold can be at high pressure when the turbo is boosting before returning to low-pressure when the throttle is released.
The one-way valve lets gas flow from crankcase to manifold and not the other way. Should the valve fail completely then the crankcase could become pressurised. From reading posts on Saabscene it looks like this pressurisation can force oil out of the filler pipe and generally do very bad things.
Was probably worth changing then 🙂