My local water, the river Lot (next big river south of the Dordogne), has some decent barbel fishing. There is also the river Cele which is much smaller, not fished it yet but have been told it is a good barbel river.
Apparently there are two types of barbel in these rivers, seems I have only caught the smaller variety so far!
Between July - September it is the holiday season. Many of the shallow barbel areas will have bathers on them well into the evening. The Cele has a lot of canoes during the holiday season!!! Early mornings or a couple of hours into darkness (if night fishing allowed!) is your best bet.