it wont kill them and they will resprout from the side shoots. if you are worried do it to just one plant see how it goes and if it is happy do the others.

I cut mine back hard after flowering and find they flower again

Mine seem to flower all summer more or less non stop so I don't cut it back until the autumn. But if yours are leggy and have only a few stems cutting back would encourage new shoots but you will lose any early flowers. Take your pick - Seaburngirl's advice is good as you can't lose.

Thank you all, I will try what you suggest, what it boils down to is this old girl is impatient yet again, I expect every thing new to be mature at birth!! This weather is brilliant warming up nicely yet the rain has not disappeared completely, what I like.