Hi M&B, We have been to both SB and GS in Sept.( usually the first 2 weeks). Weather has been great apart from odd day. We are going again on 4th Sept for another 2 glorious weeks in SB.
One thing to note, whilst days are ok, the evenings are a bit cooler and alight long sleeve top is always handy. No need to take one as you can buy them 10 a penny.

The Toonman