I'm after some advice on the best way to get from Paris CDG Airport to Paris Austerlitz station, to catch a train leaving at 11pm. We will have plenty of time (arrive CDG around 6pm). The only way I have found is RER B to Gare du Nord, then metro line 5 to austerlitz. Also, if we will have spare time, would welcome suggestions on how/where to spend it.
Chris, assuming you get to Gare Austerlitz about 8:00 pm, you will have a couple of hours to while away before you need to board your train, just enough time, really, to find a restaurant and have some dinner. You don't say what time of year this will be so there may or may not be enough daylight left for your to explore a bit. Maybe you could store your luggage and take a cab to Notre Dame, have a quick look and then find a restaurant, always keeping an eye on the time, of course.
Another route would be the RER B to St-Michel Notre-Dame and the RER C from there to Gare d'Austerlitz. Gare d'Austerlitz has both lockers and a staffed left-luggage office.