I find this plant to be hardy in this area (West Somerset) and mine came through last winter fine, so I'm hoing that the'll be OK after this last very cold weather.

All I do is snip off the dead pieces at the ends of the growth. They are all in the driest and sunniest part of my garden, and they don't like wet feet. I don't find it necessary to prune them apart from the tiny dead bits.

I hope that's enough information to help you?

I find after a few years, these can get a bit leggy and woody so my regime is to cut each shoot back by about half in spring just as the buds start to break