I would wait until early spring, and dig up the plant, and gently divide the tuber-like rhizomes. Let them dry out for a few days, and then repot or replant.

Hang on, Tug - there's only one Zantedeschia that's reliably hardy in the UK, which is Z. aethiopica 'Crowborough'.

If your Arum isn't that one, Joanie, it will need to be taken into a frost-free place for the winter. Do please check which one it is - I know you'd be upset to lose it. It has large white flowers, if that's any help.

Arrgghh! There I go assuming again! I just assumed that that was the one she had, and that it was in the ground. Sorry Joanie! I'll keep on trying.

LOL.