At some point it will be properly Autumn and I will crack out my lovely Duo knee high boots.
September can be a tricky month where the weather and temps vary across the day and height (work is in town, lower down and more sheltered, home is out of town slightly and on a hillside) and the whole sandals+jacket vs coat+boots debate is relevant. As I'm new to the area, I maybe need to be a little more aware - Cheshire is largely flat and has mild rainy weather, Derbyshire has hills and the weather appears from behind one when you were not looking.
I'm not usually a huge one for the whole wardrobe rotation thing, but definitely sandals and linen trousers can go away now, with linen jackets (sob) following after. I love my linen jackets as they come in lots more fun colours than normal office wear jackets, so I get a riot of colour all summer. However there does come a point where 3/4 sleeve orange/red/lime linen needs to be put away for next summer.
Boiled wool jacket rather like mine - I have this style in red, lime and a rich purple.