Welcome to Goy Cmw. I do mine after the flowers have gone past. I dig them up cut the flags to half the height and in an upside down V shape. Prise the rhizomes apart, with roots on each part I want to replant. I cut away any bits that do not look healthy, even if that means cutting away half of the rhizome. I then cut away any damaged roots before replanting about 12 inches apart with the top of the rhizomes showing on the surface of the soil. I believe if you have very sandy soil you can bury the rhizome. Hope this helps. I cut the foliage down to half every year after the flowers are past, always cutting in a V shape.

Yes, I do the same Scotsgran.

Thank you for the advice I have alot that needs thinning and my sister wants some of them so just wondering best time so I do not damage the plants..

As soon as the flowers are past. It gives them a chance to settle before the cold weather. Pot them in individual pots if it is not convenient to plant where you want them to go eventually. When you do plant you will be planting a pot grown plant and they can be planted at any time.