In reply to NottsRich:
Yep, that's the one, there are actually two ways to take it from there - one path goes up the hill and into the woods for about 400m distance with about 100m height gain, and this first part of the path isn't marked on the map but it's definitely there, then it turns right and descends again back to a little clearing in the woods where several paths join and the path for the cascade again climbs up the hill.
The second option follows along the edge of the road (but is separated by a bank of trees and then a ditch, so you aren't actually on the road) towards the Devouassoux bell shop (just in case you were running low on bells), past the edge of the old Grepon campsite right next to the road, and then turns left before a little stream, crossing the old campsite (watch out for travellers dogs, they all bark quite loudly but are actually quite friendly if you give them a chance) and ends up in the same clearing as the first path. From there the route is marked on the map, but it's the only path that obviously climbs up the hill, veering off to the right, towards the tunnel road.
It might give you a little bit more ascent/descent, but I prefer the first path - no road traffic, you're always in the woods, it's just a nicer setting, and it'll be a good warm up, anyway.