I would cut off the dead flower heads, and give it a light feeding. If fall holds off, you might get a second flowering, or if fall comes early, it will be all the stronger next spring.

dead head or you could reduce all the foliage by half too to see if it will give you a second flush late sept/early oct.

Next year, shorten each individual flower stem as the flower starts to go over, and you might wind up with continuous bloom as long as the weather remains warm.

Thanks for your help

Thankyou from me too. I've learnt a lot from this post so thanks to you too Fergy.