Which variety of Campanula? some are tall and narrow, others make spreading ground cover, there are quite a number of varieties. Generally, the recommended time to split those that can be split is September.

Sorry don't know the variety they are about 3 Ft high and have blue bell like flowers up the stem the mound of foliage at the bottom of the plant is about a foot in circumference

Probably easiest to dig the clump up, cut in half, then replant the sections. You can do it with some as soon as they've finished flowering, but generally safest to do it in September - the replanted parts will need to be kept well watered till late autumn.

Thanks for that I have just taken a pic I will post in my next poser so you can have a look