I did live in the UK (and Ireland as well) - and ended up not doing much indeed
Trangia: Agreed, navigation can be extremely tricky! I had a hard time not getting lost on my way to Mare aux Corneilles for instance, only to find out that the place is not worth the hassle.
Ramblin Dave: Here's a map and a list (I don't know if it's thorough) of the Dénécourt paths:
http://fabricesueur.free.fr/fontainebleau.html . I suggest the one in Franchard and Etroitures-Long Rocher... because they're the only two I've done so far. But they're great, and you can check out new areas (Franchard Raymond and Meyer and Restant du long rocher) for when you come back.
Paul Baxter: you can get this map (
http://king-detection.fr/263-pdt_480/carte-foret-fontainebleau.jpg) in pretty much any sports shop, in a bookstore or maybe even in a bar-tabac if they have a section with magazines. It's not extremely detailed though, I mean all the main paths are on it but the smaller ones probably won't be (the map would end up being too confusing I suppose). Try to stick to the paths though, there's a special type of heather in the forest that's very sensitive to any form of contact or trampling.
Saturday morning looks OK, I suggest you head early to Canche aux merciers, 95.2 or 91.1.