The message
from "grahame" contains these words:

Since I moved my Arum lilly (in a plastic pot) to stand with its feet in
my pond, a couple of years ago, it has shot up, masses of growth and
loads of flowers. I leave it there over winter. The pond is in a
slightly sheltered spot. We're in North Bucks.

So my advice leave it outside and make sure it has wet feet!

Keith

Do you think this is true of all types of lily in pots?

Not at all. Arum lily is Zantedeschia aethiopica, which isn't in the
lily family. It's just got a misleading name. The root is a rhizome. It
likes boggy wet conditions and is tender in cold areas. Mulching it in
winter is a good idea.

I just overwinter
them under a bench in a rather sheltered spot, and quite a lot die each
year. Maybe I've been keeping them too dry?

Do you mean, lilies that grow from bulbs? Possibly, they got too dry,
or froze too hard; plastic pots don't insulate very well.They are hardy
enough to be planted out in the garden, though not in the boggy wet-feet
locations enjoyed by zantedeschia.

Janet