Thanks all for your great help.
My journey has been completed and just wanted to report my experience.
It took a mighty long walk from arrival platform of Eurostar thru a complete empty Midi-station, to the departing platform of the Airport train.
At that time of night there were no ticket windows - or any other - open that I noticed.
BTW it did not seem to be a place one would like to spend any night-hours alone waiting for the next train.
As seemingly the only passenger on the deserted train, was checked for my ticket, which I luckily had purchased online.
Arrived at a completely deserted terminal around 23.45, found the only Starbucks (no wi-fi) and spend the next 6 hours accompanied by a couple of other pax that had missed their flights and some dubious homies that seemed to enjoy their permanent hang out.
Glad it worked out so well with the help in this thread and I had no need for a hotel.