Should have been done in the autumn Nellie but because they flower late in the season you'd probably get away with doing it now. Michaelmas.

as they are in a pot then it will work better than lifting them from the ground less root damage until you cut it up. an old bread knife [or the bread knife from the kitchen drawer, washed and put away without any one noticing :o) ]sawn through the plant will do the job.. make sure you firm them in well and plenty of water for agog 2-4 weeks until they look established. they may flower slightly later too.