In short, tip them out the pot. Get an old bread knife or pruning saw, and cut straight through, avoiding cutting into the shoots that stick up like little pointy columns. Job done!
Additionally, when dividing them do not cut off those long and bushy fibrous roots. If you cut them off, they will revert into a developing period to maturity (they will not show nice foliage or will not bloom either). In my experience, it will take at least two years to mature before they will bloom again (made that mistake one time). It is fun dividing plants :-)
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.