You have a choice with hardy perennials. You can leave it over the winter to protect the crown against severe frost or tidy it up in autumn by cutting it right back then. it certainly isn't too late to do it now. It will make new growth from ground level.

Thanks for the reply. I bought the plant last year and to my amazement it gave me 4 months of continuously beautiful flowers. It put on so much growth that I feel reluctant to cut it all off, but if I don't, I expect it will loose some of it's vigour and not flower so well this year.

If it worries you try leaving half of it and see what happens then. Leaving perennials untrimmed can result in the stems getting hard and twiggy looking - but nobody cuts them back in the wild after all. I found it hard to cut my penstemons back for the same reason so I compromised and cut them about threequarters back.

I think I will wait until after the anticipated snow clears before I get the secateurs out:-)