Its a bit late in the growing season to prune back this semi hardy shrub - its best to prune out spent flowerheads just behind the flower as soon as they've finished every year, which keeps the plant bushy, but if yours is quite old, you can prune out more growth after flowering next year. This gives the plant time to produce new growth and harden it off prior to autumn/winter.

Thanks for the answer. I will do as advised.