Hi Valley girl,
Sept is great usually, we have done many Septembers there, just can get chilly at night if the wind blows up, so take a cardigan / jumper in case.
No problem eating outside at night though!!
Days are beautiful, just perfect on the beach, not oppressive.
Also they often get a "Mistral" - a wind that kicks the waves up, and gives surf -
it may even be too strong to use the beach for a day, so it's pool time - but it always goes back to dead calm at some point -
it seems to go in 10 day cycles from our experience, part of the charm and a nice cycle of weather.
Quite rarely - you may get an odd storm, but they usually disappear very quickly.
There is always a little breeze onto the beach in Sept - on calm days which is lovely!