Hi Tim,
The fishing in September is OK. The main advantage is the weather is normally flat. Most any time up here you can catch fish provided you can travel to where the fish live. This is particularly so at Stanage Bay. We don't have anywhere near the seasonal changes that occur further south. If I had to make a call I would say September is poor for mudcrabs, good for mackerel and pelagics, Ok for reef fish, good for estuary fish....and lovely to be out on the water. The biggest influence on GT's seems to be tides. although I think more correctly it is locating the GT's depending on the tides, (they must be still there somewhere)
Importantly allow enough time to get the place wired. You are spoilt for choice out of Stanage and could end up going to a different Island every day for a month.I would suggest varying location and technique until something starts to work then stick to it. It is a strange place and "normal" doesn't always work.
I am not an expert on Stanage, it's about the northern limit of my patch, so not too good on exact locations.