thank you - we're thinking early September - is that still warm enough?
Most of the factors are unpredictable... But usually in September nights tend to be cold, it can be under 10 degrees Celsius at night, but it is nothing that can't be dealt with good sleeping bag (don't get go crazy with that though, it is not Siberia here... pis*ing at night can be quite a chalenge though, I mean "go out from a tent and pis* or wait till morning that is the question" ;-) ).
As for the weather it is all about luck, it can be cold and raining all the time or it can be beautifull quite warm end of the summer time with this feeling of autumn coming. But more likely mix of the two.
One thing to remember, the day becoming to feel a little short in September, the sunsets are about 7 pm then it is also becoming a little after season time. So, less people but also less things to do in the evening, some good book can be helpful.