Exactly the same time as Mrs BB and I did it in 2003 in a four and a half ton Laika and a few times the other way since in other vans. Stopped in St Lary for a few days to catch the market and then up and over. Up from St Lary to the tunnel was a doddle, down required stopping at least three times to cool the brakes. Best spot is the old (deserted) Spanish Customs post where there is some level ground. Tunnel is good France to Spain but a steady climb the other way. If the the Camping Municipal in St Lary has remained the same as it was on our last visit in 2009 it's worth the stop, all facilities at a great price and in late Sept you'll have the place to yourselves. Good luck, it's a brilliant journey.Leaving UK early September heading slowly down the east side of France so probably late September early October ish!