Hi
We've been for the last 2 years in sept, and yes it does rain some afternoons, although we were there over a week last year without any rain at all. We did get hurricane frances the first year but it only wiped out 1 full day, and central florida was up and running as normal in no time.
There were almost no queues for any rides in any parks we went to, the shops and restaurants were busy but we got in every one we went to. The cost of travelling in sept is also a big plus, we priced for next year and the difference between mid aug and mid sept is 1,000!
If DD wasn't starting big school next week, we would have booked for sep over aug next year.
I wouldn't hesitate to go in sept again.
Lynn
Edit: Just noticed you're in Millom - I'm from Whitehaven!!