In reply to StuartCJones: It can be quite dodgy. I have been there twice in late September. The first time, managed to do a few routes, then got stormed off the Meije south face around Sept 25. (No, this wasn't forecast.) After a terrible bivvy, we fought our way leftward along a ledge system to the bottom of the Carre hanging glacier, then down the normal route to the Promontoire hut. Woke up next morning to about 70 cm of snow there and it was still snowing. Sbow, then torrential rain, all the way down to La Berarde, where we were warned we should get the hell out asap lest the road be closed by a landlslide. Went back a year or two later. It had been a lousy early September and only the big valley rock routes were in nick. Tried the Pic Nord des Cavales north ridge (normally a D rock arete). Gave up after struggling up a couple of hundred metres through waist deep powder.
Then again, you might be lucky. But just as in the rest of the Alps, September snow will not usually melt.