Hi Pauline
In my experience the colours start to change from early September onwards, BUT it 100% depends on the weather over the past 12 months, and the autumn weather itself.
From mid Sept onwards, usually the colours change more each day, but holiday prices go up, and you get more and more visitors.
At some point the colours will be at their best, then there will be a storm, it will be windy, and within a day or so it's all gone. So the later you go, potentially the better the colours, but the higher the risk is of missing most of it !
Stuart