Hi, as it already has growth, I would divide as soon as possible, when they're grown outside it is normally done very early spring, just out of curiosity, why are they in the greenhouse?, they're hardy down to minus 10c, and
A aurea, and A ligtu can stand minus 15 for short periods, if you should decide to plant them outside, do it in late summer or early autumn, plant the tubers 8" deep and mulch them for the first 2 years, and a dry mulch in winter, plant in fertile, moist but well drained soil, and leave them to clump up, Derek.

Thank you for the advice Derek, I'll divide them asap. I do know they are hardy as I already have some of the tall Alstromerias in the garden, but they have spread very quickly and become a nuisance, so I kept the shorter ones in the pot. I'd put the pot in the greenhouse in November last year intending to divide them, and then forgot all about it (senior moment!) and I've just rediscovered it hidden behind the dahlia and begonia tuber boxes.