I do mine in the spring because it gives the plant room to re-grow for the next flowering time.

You can cut them back in late autumn too, but in view of the bad weather, it may be wiser to leave the top growth on as a protection against the cold. Even plants that are normally hardy have suffered this winter. I've left mine, as Hywel says.

Can be cut back from autumn to Spring, however i prefer to leave till the spring for they look great all frosted up along with all the other perennials, also if you have a large clump then divide it and increase your stock.

Thank you, Julien, Spritz and Hywel. I will leave them as they are until the frosts are over. Liz