Sam, I don't have experience on charters to Mauritania, but have used Point Afrique to and from Bamako and Niamey, from Paris. As long as you confirm your flights 72 hours ahead, as they require, then their services were fine. (The day of the flight can change plus/minus a day or so until a day or so before the flight. )
Unfortunately their web site www.point-afrique.com
says that their last charter flight to Atar from Paris is on 25 April 2004. Who knows if there will be any more.
Sorry, that is as much as I know.