I guess we don't really know if Summer is over, or indeed if it ever really started, but it's unlikely that we'll have a heatwave that warms the water too much, so the post-summer stocking has now happened. Catch Returns of Rainbows are up, and you can probably spot some in and around the big pools, especially as the water is relatively clear at the moment.
The Rainbows are not too hard to pick up on various nymphs, and some of them are putting up a great fight.
Of course you may find it much more satisfying to tempt a nice Brown to lunge at your dry fly. Failing that, the Grayling (or even Chub or Dace) can often be tempted to rise, but you have to be so quick to catch those. You will need to use small dries for the Grayling in particular, and #18 Double Badger is still my best recommendation.
Whatever you do, with the days cooling down at last, it's a great time to get down to the river.
[And please don't forget to put in your Catch Returns afterwards :-) ]
Best of Luck