I don't think a sharp knife will be strong enough Nick. It needs to be
in full growth and then using a saw, cut down through the rhizome and
rootball, making sure each piece has a good portion of roots. In order
to fit them into the new pots, you will probably have to cut about
10-15 cms. off the base of the root ball as well. Don't try to tease
the roots out, you'll do too much damage.

Allow each portion to stand for a couple of days so that the cut
surfaces of the fleshy roots can dry out and then repot into a very
gritty, loam based compost making sure the finished surface slightly
covers the base of the growths. Water well and allow the pots to
nearly dry out. Continue this until you see new leaves developing
after which, normal watering can be resumed. Plants recovering from
division need additional shade to help prevent excessive dessication,
while they re-establish their roots.