Hi Chris,
Check out this web page http://www.rafinfo.org.uk/rafescape/freteval/banknote_vickerman.htm
I promise you will find it of great interest. As well as some interesting information about Fréteval there is an image of a 100 Franc bank note which was signed by many of the men who were in the forest, including a 'N Binnie' which may have been your great grandfather?
The page also lists many of the other men who were there at the same time so if it was your great grandfather you've got a gold mine of info to get to work with. The information was supplied by a relative of one of the other men who was there at the time and I dare say that you could try and get in contact with them via the link supplied on the page.
Good luck and happy researching
Linzee