Hi...I will be arriving at the Paris Gare Du Nord station from London via Eurostar on a Tuesday, but i will only be arriving after 9pm.
So my question is where can I buy a Navigo Decouvte pass at the Gare du Nord after 9pm, and which service counter I should be going to?
Yes you should be able to buy Navigo passes after 9pm at Gare du Nord, in the south east sector of the station, on the ground/street level, there is a ticket window for "Billets Paris et Ile de France" staffed by several agents. This should be your first place to try to buy Navigo Decouverte pass at Gare du Nord. (Search for the term "Navigo passes" on that page to see a photo of the ticket window selling Navigo passes & various other Metro/RER tickets.)
The next place to try to buy Navigo at Gare du Nord is the Metro ticket window one level below street level. Follow signs for Metro 4/5 from the Eurostar train and you'll pass by this window. Again, see the Gare du Nord article and search for the term "Paris Metro ticket office" to see a photo of this ticket window.
The last place to try buying a Navigo pass at Gare du Nord is the RER B, RER D ticket window on (sub)level -2. It's at one end of the line of entrance gates before the Gare du Nord RER trains (photo) just after descending on the stairs/escalators to level -2.