Tie the foliage of the plant together loosely with string then, using a large digging fork, lift the plant. The root system is extensive, try to remove as much root, and as much soil, as possible. Then untie the foliage and split the plant using the traditional ‘two forks’ technique. The root system of a mature plant is very woody, so you will need two stout digging forks; borrow one from a neighbour if necessary rather than use a lighter border fork.
Remove the string then force one fork downwards vertically through the centre of the clump, you may need to put your weight on the fork to force it through or even hit the top of the tines with a mallet! Then force in a second fork back-to-back with the first. Push the handles apart, then together (watch your fingers) and keep working to split the plant. When it eventually breaks in two, assess the sizes of the resulting pieces. An old clump 12in/30cm across or more can probably be split into four pieces by repeating the process on each of the first two halves; smaller pieces can be left without dividing again. Write labels for each division at this point. Trim off any damaged leaves and replant at once (see page xxx). If you find the roots especially tough, use a spade to slice through the tightly congested growth.
The problem with dividing hellebores is that not only will they will often miss one or two year’s flowering but they also take a long time to recover and grow strongly again. So if a slow recovery is inevitable, why not make more than just a few plants?