When's The Correct Time To Divide Campanula Muralis Majul

I have had this plant for a couple of years now and was reading that to keep it at it's most attractive appearance, it should be divide every couple of years.

When's the best time to do the please? - RHS are saying that September is the better time to do this, whilst other sites are saying Spring time, which is the best time please?

 

Lovely picture - I have always understood that it it can be spring or autumn. In fact only last week I had to clear a patch for some pipework to be done and I actually dug up a large quantity and transplanted it. 10 days later still looking healthy and still flowering.