In reply to NaomiHart:
Sorry, I won't be going there this weekend, but I have a couple of suggestions.
You're planning on getting line 381 to Millau from Montpellier?
After that you can get a bus from Millau to Le Rozier:
http://www.keolis-aveyron.com/LesAutocarsKeolis/Leslignesinterurbaines/Lesl...
Le Rozier is at the junction of the Tarn and Jonte Gorges, this is only about 15km from the campsite and pretty much every car/van/motorhome will be going at least to Les Vignes, so you should be able to hitch up the Gorge quite easily.
The other option is to try and get a lift from local climbers. There's a bouldering wall in Millau which is where most of the locals hang out. It's a bit out of town, at 1 rue de la prise d'eau. Their opening times are
Vendredi : 17h00 - 21h00
Samedi : 9h00 - 12h00 / 13h30 - 19h00
and you can usually buy the topo from them too.