I cut mine down to the ground but not until the spring. Otherwise I cut halfway as new buds can fom if the weather is clement.

Thank you Arbuthnot, I'll cut them halfway for now. I will have to thin them out as well because they are spreading. hadn't realised they did that.

I do half way too and then in the spring remove the stems that have totally died back. the stems also provide a home for overwintering insects etc.

Thanks to all. I will do just that!