Posted: 13/04/2015 at 17:41
Take them out of their pots and us an old bread knife to saw them into 4 even sized chunks then repot using JI no 3 compost. Keep them watered as usual and they'll grow away quite happily. Give them a top dressing of slow release food every spring until it's time to divide and pot again.
I've just done this with one huge 60cm wide potted Sum and Substance that ended up giving me 10 new plants and another one whose label I have lost that gave me 8 plants. About to do another 40cm pot now that it is showing its noses and will get at least 4 from that.
If you haven't room for all the plants produced, swap them with neighbours or friends. I swap mine with friends and also sell some at a charity plant fair I help organise every May.