After flowering is the best time and then you can prune hard, taking out old flowered stems, woody ones and weak ones. New canes will grow from ground level.

Ojibway's right, after flowering is the best time. You can do it now, but you will lose flowers next year. If its spreading too much, dig out a clump and discard - it does tend to take over the garden if you're not careful.

hi thanks it has really taken hold this year i usually prune it back often but as you say it does affect the flowering cheers pete