You can do it now. When it grows back it may even bloom again.

Best to prune it right after bloom, anyway. In my experience they can take severe cutting back just fine. If you're really worried, just cut a few of the longest branches back to the base, cut a few more next summer, and finish the job the third year.

Thank you both very much. I will have a go at it as soon as the rain stops! We have been crying out for rain since April and now at last it's here. I think I will do as you suggest, Tug, and start by taking out some of the older stems and cutting the rest back quite hard.