Apparently there will be shuttle buses to the train station which is only circa 15 - 20 minutes away. Actually you could walk there in about that time but there is a main road to cross

I was thinking of going on the train (you can drink then

I guess I don't mind sleeping on the station if it comes to that

Driving is an option but I suspect that a lot of people will do that and getting out of the car park would be hellish.
In terms of weather, who knows? Early September can be really great. Admittedly it could also be cold and raining.