It's disconnected on my car. Should the hose be plugged back in? If not, should it be plugged as it isn't currently?
It should be connected under the inlet manifold manifold off a small vac nipple. If its not then you need to find out whats on that nipple
Lets get the terminology straight here... by 'combi' valve I am assuming you mean the grey valve in the centre of your pic above... if so this is the N249 valve
The valve just needs to be plugged in electrically (as it is currently)... no need to pipe this up at all as this is now 'by passed'... just cable tie it out the way..
<tuffty/>
You beat me to it! Just discovered that from here:
http://www.audi-sport.net/vb/general-technical-how/84402-n249-bypass-how-2000-s3-but-same-many.html
So what's its use? When you say "bin it off", how? Remove it from the car completely? What's the benefit/downside to doing so?