Mid October will probs be best Stan. Much depends on the preceeding weather though, with lots of sunshine (like we're getting now) followed by cold nights providing the best colours. If we have another stormy couple of weeks in mid october though it tends to ruin everything.
Oh, and I wouldnt go at the weekend if I was you, I have tried before to get in when the weather was good on a Sunday and the queue of traffic was all the way back to Tetbury!
Top Tip: Get there for opening midweek or at the wekend and you will often find that they a) open a bit early and b) dont have staff to releive of your cash til about an hour later
Give me shout when you are planning on going and I will see if I can join you!