by too large do you mean the clump? if yes then lift, separate and replant immediately. they have quite brittle roots and resent disturbance. Firm the soil around them firmly to prevent 'wind rock'. keep them well watered and cut off any faded flowers and last years old leaves if you didn't remove them during flowering.

Having said that, the optimum time for moving and splitting them (the x hybridus types and H. niger) is as the soil begins to cool down, so September onwards.

that's very true Owd though I have done this about now as I had no choice to but to move as we were having work done.