In reply to philipivan:
The tunnel past the Chambon dam is still closed. There has been a back-up road created on the opposite side of the reservoir (for cars only) which takes a bit longer to negotiate but enables the valley to be travelled.
I had heard this but wasn't sure if it was always open or if it would be a good idea for a non local. I guess it might take a bit longer than usual anyway so maybe one of the diversions would be more sensible?