Rob,
looks good, so thats on my next visit to Pairs too, and being close to the capital, looks like an easy day trip.
Here's the website
http://museedelagrandeguerre.eu/en
By the way, if you have another day spare goto Compiègne, a hour out of Gare de Nord, and goto the
Musée de la Figurine Historique (Toy Soldier Museum) where they have 100,000 toy soldiers on display. I took about 2 1/2 hours to view them, and that was at a resonable pace.
http://www.musee-figurine.fr/
As well you can goto the site where the Amistice was signed in 1918 the carriage in the forest nearby, and so its well worth the trip. We stayed in Compiègne and left the hire car there, before training into Paris on the end of our holiday last year.
John