I would dig them up after flowering, split them, add feed and then bury them deeper. That will help stop the stems following over.

If they're not in full sun, they have a tendency to reach forward to get more light, and that's often the cause of this problem. Yes, you can cut the flowers off now, it won't impact on next year's flowering.

Thanks very much to both of you, I think I will have to split the plant as it has grown so massive, even though I know they like their roots to be restricted. And I will cut off the flower spikes now. Again, thanks for your help.

I'll say it again - its a myth about restricting the roots - hardy varieties grow and flower perfectly well in open ground, Valatwell, so don't worry about it.