Posted: 19/06/2012 at 14:13
@Ross Gravett: sorry - seem to have rather got away from the original question you posed. It seems to me that these plants do have more than the traditional one year's flowering in them, so your question about dividing them perhaps needs some consideration.
Not having done it, I can only theorise. But the good rule of thumb is to remember that plants are at their weakest when in flower if you are wanting to divide/move them. So I think I would wait until they had finished flowering (and dead-heading them will give many weeks of blooms) and then give them a bit of a feed. Come the autumn, when the soil is still warm, dig them up, divide and re-plant, watering in generously, but making sure that the roots have good drainage under them. (Add grit if your soil is heavy).
It must be worth a try - after all, what have you got to lose?