They are better left till spring.Even though they are hardy,its still better to be cautious,and put a mulch around the roots,in case of extreme weather.If you prune now,the cuts will be exposed to the elements.Wait till you see growth
from the bottom in spring,and then prune

I have always done mine in late march early april. less likely to be killed by the frosts.

Oh yes....I agree with Bloomer and Seaburngirl....I'm in Northumberland and wait till I see signs of new growth before I cut back hard.......could be March...,could be end of April.....